# ArcaNN: automated enhanced sampling generation of training sets for chemically reactive machine learning interatomic potentials

Rolf David, Miguel de la Puente, Axel Gomez, Olaia Anton, Guillaume Stirnemann, Damien Laage

► To cite this version:

Rolf David, Miguel de la Puente, Axel Gomez, Olaia Anton, Guillaume Stirnemann, et al. ArcaNN: automated enhanced sampling generation of training sets for chemically reactive machine learning interatomic potentials. Digital Discovery, 2025, 4 (1), pp.54-72. 10.1039/d4dd00209a. hal-05345271

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# ArcaNN: automated enhanced sampling generation of training sets for chemically reactive machine learning interatomic potentials†

Rolf David, ID* Miguel de la Puente, ID Axel Gomez, ID Olaia Anton, ID Guillaume Stirnemann* and Damien Laage ID*

The emergence of artificial intelligence is profoundly impacting computational chemistry, particularly through machine-learning interatomic potentials (MLIPs). Unlike traditional potential energy surface representations, MLIPs overcome the conventional computational scaling limitations by offering an effective combination of accuracy and efficiency for calculating atomic energies and forces to be used in molecular simulations. These MLIPs have significantly enhanced molecular simulations across various applications, including large-scale simulations of materials, interfaces, chemical reactions, and beyond. Despite these advances, the construction of training datasets—a critical component for the accuracy of MLIPs—has not received proportional attention, especially in the context of chemical reactivity, which depends on rare barrier-crossing events that are not easily included in the datasets. Here we address this gap by introducing ArcaNN, a comprehensive framework designed for generating training datasets for reactive MLIPs. ArcaNN employs a concurrent learning approach combined with advanced sampling techniques to ensure an accurate representation of high-energy geometries. The framework integrates automated processes for iterative training, exploration, new configuration selection, and energy and force labeling, all while ensuring reproducibility and documentation. We demonstrate ArcaNN's capabilities through two paradigm reactions: a nucleophilic substitution and a Diels–Alder reaction. These examples showcase its effectiveness, the uniformly low error of the resulting MLIP everywhere along the chemical reaction coordinate, and its potential for broad applications in reactive molecular dynamics. Finally, we provide guidelines for assessing the quality of MLIPs in reactive systems.

Received 30th June 2024
Accepted 21st October 2024

DOI: 10.1039/d4dd00209a

rsc.li/digitaldiscovery

## 1 Introduction

The advent of artificial intelligence has revolutionized many fields of science, and machine learning has become an essential part of the scientific toolbox. In computational chemistry, machine-learning interatomic potentials (MLIPs) now offer an attractive method that combines accuracy and efficiency for

calculating atomic energies and forces, which are the computational bottleneck when running molecular simulations. They have already led to remarkable success, ranging from the simulation of very large-scale systems¹ to phase diagrams and transitions,²⁻⁴ metallic melts,⁵ interfaces,⁶⁻⁹ proteins in explicit solvent,¹⁰ and chemical reactions.⁶,¹¹⁻²⁰

MLIPs provide a very high-dimensional fit of the potential energy surface (PES) of the system of interest, mapping the configuration space onto the potential energy. Most of the computational cost is incurred *a priori* during the training of the model on a dataset that spans the range of important molecular structures.²¹⁻²⁶ The subsequent trajectory propagation then involves a much less expensive evaluation of forces with these potentials. This therefore contrasts with other molecular dynamics methods which determine forces on-the-fly *via* costly calculations involving, *e.g.*, electronic structure determinations, that need to be repeated for each configuration encountered along the trajectory.

Over the years, a considerable effort has been devoted to the optimization of algorithms and network architectures, ranging from kernel-based methods²⁵,²⁷⁻²⁹ to high-dimensional neural networks and their many flavors.³⁰⁻⁴⁶ As a result of these recent developments, MLIPs now offer an attractive alternative to DFT-

PASTEUR, Département de Chimie, École Normale Supérieure, PSL University, Sorbonne Université, CNRS, 75005 Paris, France. E-mail: rolf.david@ens.psl.eu; guillaume.stirnemann@ens.psl.eu; damien.laage@ens.psl.eu

† Electronic supplementary information (ESI) available: Details of the initial *ab initio* MD simulations for the S<sub>N</sub>2 reaction, timings of ArcaNN training for the S<sub>N</sub>2 and Diels-Alder example reactions, examples of user-provided folder structures, examples of machine JSON files used by ArcaNN, examples of JSON control files generated by ArcaNN, evolution of candidate and rejected structures with exploration time for the ArcaNN reactive training for the S<sub>N</sub>2 reaction, joint density distribution of key distances in training datasets for the S<sub>N</sub>2 reaction, validation of the R5 NNP for the S<sub>N</sub>2 reaction and R8 NNP for the Diels-Alder reaction, free energy profiles and collective variables for NR2 and NR3 NNPs for the S<sub>N</sub>2 reaction, OPES 1D free energy profiles and CVs from the R5 NNP for the S<sub>N</sub>2 reaction, and joint density distribution of key distance in the umbrella sampling simulations for the Diels-Alder reaction. See DOI: https://doi.org/10.1039/d4dd00209a

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based<sup>47,48</sup> and reactive force field<sup>49</sup> molecular dynamics simulations. While their computational cost is only moderately larger than that of classical force fields, they can be trained on high-level reference electronic structure calculations that provide much greater accuracy than empirical force fields. Their efficiency is thus many orders of magnitude greater than that of DFT-based simulations.

However, while recent advances have considerably optimized the architecture of MLIPS and their descriptors, dataset construction – another critical aspect affecting the quality of their energy and force predictions – has not been as extensively explored. Indeed, the training dataset should sample all typical configurations that will be encountered during the simulation, while avoiding redundancies.

Different strategies have been adopted for the construction of the training dataset, depending on the type of processes to be simulated and on the available data. In the first approach, the MLIP is trained only once, on a large collection of already available structures. This is the case, for example, with the general-purpose potentials ANI<sup>34,35</sup> and MACE,<sup>50</sup> which are trained on a large dataset of chemically diverse organic molecules in their equilibrium geometries. The resulting potential can then successfully describe the equilibrium fluctuations of a wide range of compounds in the gas phase. However, larger geometric distortions that exceed the amplitude of thermal fluctuations are not included in the training dataset and are likely to be poorly described by the MLIP.

A type of active learning approach based on successive iterations, named concurrent learning,<sup>51</sup> has thus been proposed. Starting from an initial dataset, a first generation of MLIPs is simultaneously trained. The latter are then used for exploration of the potential energy surface *via* unbiased molecular dynamics simulations, possibly under various temperature and pressure conditions. In the configurations that are encountered, the quality of the MLIP prediction is estimated using a query-by-committee approach,<sup>52</sup> which measures the deviation among the predictions of the assembly of potentials that were trained on the same dataset (but with different random initializations). Configurations in which the prediction uncertainty between the committee is large are then labeled with the reference calculation method and added to the training dataset for the next iteration of training and exploration. This approach is, for example, successfully implemented in DP-GEN<sup>53</sup> and expanded in ChecMatE.<sup>54</sup> We also note that recent uncertainty-aware and uncertainty-driven techniques have emerged as powerful tools for enhancing the accuracy and efficiency of MLIPs.<sup>55-59</sup> By calculating the uncertainty of the MLIPs compared to the reference method, selecting configurations with high uncertainties, and possibly biasing the exploration of configurations toward poorly described regions, these approaches optimize the learning process, leading to more reliable and robust MLIPs, particularly in materials science. Other recent strategies, such as data distillation,<sup>60</sup> have started to address the key component of constructing the training dataset.

However, a particular challenge is posed by chemically reactive systems, which require an accurate description of the energies and forces everywhere along the chemical reaction coordinate, including in the vicinity of high-energy transition

states that are very rarely sampled spontaneously. This difficulty is well known,<sup>61</sup> and has started to be addressed by some initial efforts. A recent study<sup>62</sup> has proposed a general-purpose reactive MLIP in condensed phases trained on a dataset including configurations collected over a wide range of temperature and pressure conditions. Although this potential was shown to be successful for a number of chemical transformations, its exploration remains limited by the regions of the PES accessible *via* temperature and pressure changes, which implies that it is not adequate for chemical reactions with large energy barriers. Another effort<sup>63</sup> specifically sampled reaction pathways but was limited to reactions in the gas phase. In a different approach, the training dataset can be enriched with configurations generated by enhanced sampling techniques,<sup>64</sup> by performing random infinitesimal displacements,<sup>65</sup> or by a combination of transition tube and normal mode sampling.<sup>66</sup> In a very recent study, a combination of uncertainty-driven dynamics and enhanced sampling was proposed to address reactivity at solid interfaces.<sup>67</sup> All these strategies aim to explore the high-energy regions of the PES. However, there is still a crucial lack of standardized procedures. A set of uniform and consistent protocols would be needed to ensure that the training is easily reproducible, with proper bookkeeping of every file and parameter, as well as a computational platform and workflow to support this. Currently, each user must either manually or semi-automatically implement their own strategy, which becomes increasingly tedious for more complex systems, as constructing a reliable dataset typically involves many iterations.

Here, we address this major challenge for the efficient simulation of condensed phase chemical reactions. We present ArcaNN, a comprehensive framework for generating training datasets for reactive MLIPs. It combines a concurrent learning approach for the controlled convergence of the potential and a wide range of advanced sampling techniques for exploring the chemically relevant configurations, including high-energy geometries. The exploration dynamics can be performed with either classical or quantum nuclear dynamics. These successive steps are integrated into an automated approach that includes training, extended exploration, new configuration selection and associated energy and force calculations at the reference level (labeling) steps, while keeping records so that the procedure can be easily documented and replicated.

In the following, we first summarize the main steps of concurrent learning for MLIPs and describe the ArcaNN code, its architecture, and the different steps involved in the iterative training dataset generation. We then illustrate its capabilities on two paradigm reactions: a nucleophilic substitution in solution and a Diels–Alder reaction in the gas phase. We finally provide some concluding remarks about the applications and future developments of our code.

## 2 Design of neural network interatomic potentials: overview

The objective of MLIPs, represented in Fig. 1A, is to approximate the potential energy surface (PES) of a system. For details

<figure><img src="image_4.png" /></figure>
<figure><img src="image_5.png" /><figcaption>Fig. 1 (A) Schematic representation of a neural network potential (NNP) that approximates the potential energy surface (PES) of a system. With a molecular structure as input, the NNP predicts the energy and forces of the system. (B) Schematic representation of the iterative training of NNPs using a concurrent learning loop. During training, several NNPs are trained on a dataset of molecular configurations, each labeled with their corresponding energies and forces. During exploration, they are then used to run MD simulations and the selection phase assesses whether there are new candidates to be labeled to expand the datasets. The loop between training, exploration, and labeling can be repeated multiple times until there are no more candidates and the NNP is then deemed ready for production. In ArcaNN, the exploration phase is improved by the use of enhanced sampling techniques to explore the chemical phase space (C) and the possibility to perform path-integral MD simulations (D). This allows the iterative enrichment of the dataset, leading to a complete description of the chemical reactivity (E).</figcaption></figure>

regarding the different types of MLIP architectures, the training and choice of descriptors for the atomic environment, we refer the reader to excellent reviews,⁶⁸⁻⁷⁴ of which we provide a brief overview below.

MLIPs have been developed based on different types of architectures, including artificial neural networks and kernel-based methods.²⁵,²⁷⁻²⁹ A breakthrough in neural networks potentials (NNPs) came from the high-dimensional neural networks (HDNNs) introduced by Behler and Parrinello.³⁰ The total energy of the system is decomposed into a sum of atomic contributions, which are assumed to exclusively depend on the local atomic environment encoded by a descriptor that satisfies the required PES invariances. Two key advantages of this scheme and of this locality approximation are their computational efficiency and the possibility to extend these neural network models to arbitrarily large systems. HDNNs with local descriptors based on a cutoff radius around each atom are used in several implementations, including BP-NNP,³¹⁻³³ ANI,³⁴,³⁵ and DeePMD.³⁶,³⁷ Other MLIPs use the same atomic decomposition of the total energy but employ invariant message-passing neural networks (MPNNs)⁷⁵ for their descriptors; these implementations include, e.g., DTNN,³⁸ SchNet,³⁹ PhysNet,⁴⁰ and HIP-NN,⁴¹

which can access non-local information beyond the cutoff radius. Recent improvements include the use of equivariant, atom-centered, message-passing neural networks, like NequiP⁴² and its evolution Allegro,⁴³ which have been suggested to provide an improved accuracy compared to local approaches, and to remove the limitations on accessible length scales. Finally, local models can also be extended by adding higher-order terms describing long-range effects and interactions.⁴⁴⁻⁴⁶,⁷⁶,⁷⁷

NNPs are trained using a supervised learning approach, on an ensemble of molecular structures, each labeled with their corresponding energies and forces. They usually demonstrate excellent accuracy in interpolating, i.e., predicting energies and forces for new configurations close to those seen during their training. However, this accuracy drops dramatically when extrapolating to configurations not seen during training, which is a key issue in machine learning models. For molecular dynamics simulations, this implies that if the trajectory ventures outside of the training dataset region, the NNPs will typically lead to unphysically large forces that abruptly terminate the simulation.

This issue can be addressed by identifying all relevant configurations *a priori*, for example, from an extensive sampling with a long simulation. However, this requires the ability to calculate the energies and forces during this long trajectory and necessitates, for example, *ab initio* molecular dynamics (aiMD). This solution is not practical since sampling with aiMD is computationally demanding, especially when the configurational space to be mapped is large. In addition, propagating long trajectories with good accuracy for the force calculations is precisely the objective of NNP-based simulations.

To address this situation where the volume of unlabeled data can be large but the cost of labeling is high, an iterative construction of the training dataset inspired by the concept of active learning⁷⁸ was proposed to navigate through the data, gather feedback, and proactively seek labels for data points that are marked as requiring further attention. This concurrent learning approach,⁵¹,⁵³ illustrated in Fig. 1B, involves three main steps: exploration, labeling, and training. These steps are repeated until convergence, which can be estimated using various descriptors and criteria.

However, exploration trajectories are usually propagated without any bias in the configurational space, and, as a consequence, chemical reactions with a free-energy barrier exceeding a few times the thermal energy do not spontaneously occur on the timescale of these simulations. An additional limitation is that during a reactive trajectory, the time spent in the transition state region is very limited. This unbalanced sampling therefore contrasts with the objective of achieving uniform sampling along the reaction coordinate to ensure that the error is low everywhere along the reaction path. Another limitation is that chemical reactions are rare events, and a given reactive trajectory between reactant and product regions is often short-lived (on the picosecond timescale). Finally, another difficulty is that for systems where several reaction pathways are in competition,¹⁴,¹⁵ we would like to sample all pathways and not only the minimal free energy one.

<figure><img src="image_6.png" /></figure>
In order to better sample high free-energy barriers, enhanced sampling simulations are necessary. Examples include, but are not limited to, umbrella sampling,⁷⁹ metadynamics⁸⁰ and its variants,⁸¹,⁸² which have already been successfully applied in the context of data generation for NNPs.⁶,⁸,¹³⁻¹⁵,²⁰,⁶⁴,⁸³ Generally, these approaches require identifying a set of collective variables (CVs) to bias the exploration trajectories, or setting up multiple enhanced sampling simulations covering numerous CVs to ensure that the reaction pathway is sampled adequately.

An important limitation in the current state of the art is therefore that users must either resort to a nano-reactor approach,⁶² which sacrifices control over specific reactivity and pathways, or they must manually set up numerous enhanced sampling simulations, which are both tedious and time-consuming. This is the limitation addressed by ArcaNN. It provides a comprehensive, flexible and automated workflow to generate datasets to train reactive NNPs while recording all the steps leading to the construction of the datasets, which can thus be easily shared and reproduced, a step towards meeting the FAIR principles⁸⁴ for research data.

# 3 Streamlining the construction of a reactive dataset with ArcaNN

## 3.1 Concept

ArcaNN is a comprehensive framework, interfaced with other neural network, molecular simulation, and quantum calculation software for training NNPs, propagating trajectories, and labeling new configurations. ArcaNN allows the sampling of the chemical phase space of a given reaction (encompassing reactants, products, intermediates, and transition states with the solvent treated in a reactive manner) to adequately and efficiently build a dataset that can be used to train NNPs.

The workflow combines the concurrent learning approach with enhanced sampling techniques, as shown in Fig. 1C. Starting from an easily generated dataset of structures in the reactant and product regions, ArcaNN supervises the simulation of either classical or path-integral swarms of short biased dynamics. The dataset is progressively enriched with representative structures along the reaction pathways, on which generations of NNPs are iteratively trained and used for successive rounds of exploration. This approach not only makes more efficient use of computational resources compared to an equivalent biased initial *ab initio* trajectory but also provides a greater number of uncorrelated samples, leading to more accurate NNPs.

## 3.2 Overview of the code and definitions

ArcaNN is a Python 3 package designed in a modular fashion to facilitate its extension, modification, and the integration of new features, such as interfacing with new software or types of MLIPs. The current version of ArcaNN is interfaced with the following programs:

* CP2K⁸⁵ for labeling.

* DeePMD-kit⁸⁶,⁸⁷ for training the NNPs.

* LAMMPS⁸⁸ or i-PI⁸⁹ for exploration using the DeePMD NNPs, both in combination with Plumed⁹⁰ for enhanced sampling.

ArcaNN maintains a clear and easily readable record of the workflow. This framework offers great flexibility at each workflow step, including the full range of quantum chemistry methods available in CP2K and the diverse enhanced sampling techniques and CV definitions offered by Plumed. Users can also choose to explore any number of systems. As detailed below, these correspond to a combination of MD parameters, thermodynamic conditions, and chemical compositions.

ArcaNN is specifically designed for High-Performance Computing (HPC) clusters with CPU and GPU resources, exploiting them in an embarrassingly parallel fashion. It utilizes VMD⁹¹ for trajectory manipulation in the DCD format and Atomsk⁹² for converting LAMMPS data files to and from the XYZ format.

From the initial datasets and user-provided files, ArcaNN oversees the creation of necessary files and folders for the interfaced programs and submission scripts for HPC resources. It manages the training of NNPs, the exploration of phase space, and the labeling of configurations, and it iterates these steps until the NNPs accurately describe the targeted reactivity of a given system. While requiring minimal intervention, ArcaNN gives users full control over the iterative process through a series of steps and phases whose parameters can all be set or modified before execution. We now describe, in the next 4 sections, the concepts of steps, phases and systems around which ArcaNN is organized as well as address what user files are needed to start the ArcaNN procedure.

### 3.2.1 Steps.
ArcaNN's architecture is structured around five modules (each corresponding to a step in the concurrent learning scheme, see Fig. 1B): initialization (1), training (2), exploration (3), labeling (4), and testing (5) (Fig. 2). Each step is further divided into a succession of phases, which are detailed below.

### 3.2.2 Phases.
A phase is a subdivision, representing a specific execution of ArcaNN within a step, and corresponds to

<figure><img src="image_7.png" /><figcaption>Fig. 2 The ArcaNN's architecture divided into five main modules corresponding to the successive steps: initialization, training, exploration, labeling and an optional test module. Each module is divided into several phases that are executed in sequence with user intervention, either proceeding from top to bottom within the same module or by following the arrows between modules. Phases with an asterisk (*) invoke a scheduler to submit the resource-intensive jobs to the HPC, while the others are almost instantaneous and are executed on the login node. In green are the phases that are mandatory, in orange are the optional phases and in blue are the phases that are mandatory only in the case of path-integral MD simulations.</figcaption></figure>

<figure><img src="image_8.png" /></figure>
the command that the user executes. The outcomes of each phase within a step are stored in JSON files in a control folder, easily readable by the user. This ensures the traceability of the workflow, and allows retrieving information in an automated way. In particular, the status of each phase within a step is recorded and checked, avoiding the risk of skipping a non-optional phase or executing them in the wrong order. In addition, from iteration to iteration, if no user input is provided, parameters are propagated or re-calculated automatically.

ArcaNN requires minimal user input beyond the user files detailed below and a comprehensive manual accompanied with example files is provided on the GitHub repository.⁹³ ArcaNN generates all the necessary files and directories for the workflow; its operating parameters are set to default values unless tuned by the user as needed. Each time a phase is executed, two JSON files are created. One is the default JSON (default_input.json), where all the default values used are stored, providing guidelines for the user. The other JSON (used_input.json) stores all the values used for this specific phase and is created only if the phase is successfully completed, ensuring the traceability of the values used for each phase in each iteration.

Any default value can be modified through the user-provided JSON file (input.json). For example, if a user executes the training prepare phase but wants to change the learning rate, they can provide an input.json containing only the learning rate value. The user will then relaunch the training preparation phase and the input.json values will be taken into account. In this scheme, the priority is given to user values, then to values used in the previous iteration or auto-calculated from the previous iteration, and finally to the default values. This is useful, for example, if the user wants to change a parameter in the exploration; this change will be carried over to the next iteration without the need to provide an input file again. If a phase fails, an explicit message will be displayed, and the user will have to fix the issue before re-executing the phase.

### 3.2.3 Systems.
A training dataset for a MLIP should be representative of the chemical phase space, and can include configurations with different chemical compositions, different thermodynamic conditions, and different exploration biases. In ArcaNN, this is described by systems. Each system is characterized by its chemical composition (e.g., reactant at different concentrations), thermodynamic conditions (temperature or pressure for example), whether the exploration is carried out with classical or path-integral MD and, if desired, the type of biased sampling along pre-determined CVs that will be executed. These systems are defined by the user and will be the core of the exploration phase, crucial for the generation of the training dataset. For example, in the process of building a reactive dataset to describe a given chemical reaction from A to B, the user can configure twelve systems: (1) unbiased MD simulations of the reactants; (2) unbiased MD simulations of the products; (3) MD simulations starting from the reactants using On-the-fly Probability Enhanced Sampling⁹² (OPES) along one or several CVs that could be good reaction coordinates (RCs); (4) the same simulations starting from the products; (5) steered MD simulations⁹⁴ along similar coordinates, transforming the reactant state into the product state; and (6) the

opposite, from the product to the reactant. Then, each of these six setups (1–6) could be executed at two different temperatures: 300 K and 325 K, leading to a total of twelve systems.

Another feature of ArcaNN is its flexibility: the practical number of systems a user can define depends on their available HPC resources, rather than being constrained by the ArcaNN methodology itself. Importantly, the systems do not need to have the same chemical composition. For instance, one might include a system composed of one set of reactants and another that simulates a higher concentration with two sets of reactants, possibly within a larger solvent box. Furthermore, systems can be constructed from different molecular configurations, such as one with reactants and products, and another with reactants and different products, representing competitive reactions, or even varying solvents to explore a wide range of chemical environments.

### 3.2.4 Required user files.
To start the ArcaNN procedure, users should provide two sets of files. A first set of files corresponds to the user files, which are organized in a user_files folder, with a minimal folder structure as shown in Fig. S1.† We provide skeleton user files in the GitHub repository⁹³ that users can use as a template to create their own files and interested users are encouraged to refer to the documentation for details about these files, including which parts of each file are important to retain so that ArcaNN can read them and auto-fill the needed values. This choice was made to ensure that users have full control over the files and can adapt them to their needs, as well as to ensure that the framework remains as flexible as possible. These include the various inputs of the external software used in the workflow, such as CP2K, DeePMD-kit, LAMMPS, i-PI, and Plumed, together with the job scheduler files needed to submit the external software jobs. It is important to note that, except for the training step, users should provide one input file for each system they wish to simulate, i.e. one LAMMPS (or i-PI) input file, one Plumed input file (if needed), one LAMMPS datafile, and one CP2K input file per system. One LAMMPS datafile is needed per system to define the initial configurations to be simulated. LAMMPS datafiles are preferred since their format is standardized and more flexible than XYZ files. A properties file is also needed to specify atomic types and masses.

To control the use of HPC resources, ArcaNN uses a machine.json file where HPC resources are identified through a keyword, and the configuration outlines various attributes of the HPC machine, such as the job scheduler, the maximum number of jobs in the queue, and the maximum scheduler array size. Furthermore, it provides specifics for project or task setups within this HPC resource under sub-keyword, including names for projects and allocations, architecture type, and a designated partition for job queuing as well as valid tasks for execution. Importantly, it facilitates the incorporation of multiple HPC machines, for executing specific tasks on GPUs and others on CPUs. An example of a machine.json file can be found in Fig. S2,† and more details can be found in the documentation on the GitHub repository.⁹³

A second set of files required to initiate the training process corresponds to the initial training dataset. In the current

<figure><img src="image_9.png" /></figure>
version of ArcaNN, these datasets, which include atomic configurations, energies, forces, and virial tensors, should be formatted in the DeePMD-kit format.

We pause to provide some useful guidelines on how to generate these datasets. They are typically obtained from short aiMD simulations. To enhance the training efficacy, it is recommended that these datasets contain as many uncorrelated configurations as possible, primarily spaced over time. As a rule of thumb, configurations spaced by 20 fs provide a good starting point.

If the aiMD simulations are performed at the same DFT level as the desired reference for the NNPs, the user can directly supply the associated energies, forces, and virial tensors. However, to improve the computational efficiency, a recommended practice is to conduct the aiMD at a less computationally demanding level of theory before executing the reference level calculations solely on the selected configurations. This approach is advantageous, as the geometries generated by a cheaper theory level are usually reliable, but the energies and forces are not as good as those provided by a higher level description. For instance, initial simulations can employ a GGA functional with a minimal basis set, while subsequent reference calculations use a higher level GGA or hybrid functional accompanied by a larger basis set. Alternatively, users may opt for even more cost-effective calculations, such as semi-empirical methods like DFTB2 (ref. 95 and 96) or GFN2-xTB,⁹⁷ and then perform the reference calculations on the selected configurations. ArcaNN offers flexibility in managing the initial datasets, including the option to discard them if their energy distribution significantly deviates from that of the datasets constructed during the iterative training process. Moreover, it accommodates the addition of extra datasets, independent of the initial and iterative ones, at any stage of the training. This feature is particularly useful if users provide datasets from other sources or systems that they wish to incorporate. For example, as initial datasets, users can provide datasets sampling the reactants, the products, and the pathways from reactants to products, as well as datasets from products to reactants obtained from aiMD.

## 3.3 Workflow

As shown in Fig. 2, the workflow is divided into five main steps: initialization, executed once at the beginning of the workflow; training (of the NNPs); exploration (swarms of enhanced sampling trajectories with selection of candidates); labeling (labeling the new candidates with the reference method), which are integral parts of the concurrent learning cycle; and testing, which is optional and can be used to assess the training of the NNPs. A recurrent phase is the optional clean phase that can be executed at the end of each step to remove unnecessary files, such as temporary files created by ArcaNN and redundant files. The other phases are specific to each step, and are detailed below. The next sections will describe the different steps and phases of the workflow.

It is important to note that the execution of these steps is not automated; each phase must be manually initiated by the user.

While resource-intensive tasks, such as training, exploration, and labeling, are submitted to the HPC queue manager (e.g., SLURM) for execution, ArcaNN does not provide automatic updates on their completion. Instead, the user should manually check the status of these tasks in the corresponding check phases before moving on to the next phase. This method requires more user involvement but ensures precise control over the workflow and facilitates troubleshooting and adjustments based on intermediate results.

### 3.3.1 Initialization.
The first step of the workflow is the initialization step, which is executed only once at the beginning of the workflow. It consists in one user set-up phase and an initialization start phase. To initiate the process, users are required to supply a set of initial files to ArcaNN (see above), which are used to generate all the files and directories needed for the subsequent training, exploration, and labeling steps. After this initial set-up is completed, no additional user-provided files are needed.

When the set-up is complete, the user can proceed to the initialization step which involves a single phase, start, ensuring the presence of all the user files. This step corresponds to the creation of the initial training folder and the control directory, where the JSON files are saved. Additionally, it locates the initial datasets and tags them for the first training step. This phase also reads all the names of the LAMMPS datafiles provided by the user and then automatically creates the list of systems that ArcaNN will use for the exploration and labeling steps. In this step, the user can also choose the number of NNP models to train for the committee, which is set to three by default. After this step is successfully completed, the user can proceed to the training step.

### 3.3.2 Training.
This section describes the training step. The goal of this step is to train a generation of NNPs on the current dataset, and to prepare them for the exploration step.

During the training step, a committee of several NNPs are trained based on the existing structures in the current dataset. This step is divided into the following phases: prepare, launch, check, freeze, check_freeze, compress, check_compress, increment and clean, with an overview of the phases represented in Table 1.

The prepare phase will create the necessary folders and files for the next phase. It will copy the datasets, and the dptrain.json (which is the DeePMD-kit input) file to the training folder and we refer to the documentation of the DeePMD-kit⁸⁷ for this file and the associated hyperparameters. In this phase the user can define, for example, the learning rate, the number of steps, and the machine keyword for the job scheduler parameters (for more details, see the documentation on the Github repository⁹³).

All the subsequent phases do not require further user inputs. After the prepare phase, the launch phase will submit the training jobs to the HPC cluster. The check phase will check if the training is successful, and will provide guidelines about the training duration that can be used for the next iteration. The next phase, the freeze phase, will submit jobs to the HPC cluster to convert (i.e., freeze) the models from their trainable parameters (e.g., weights and biases) to constants and remove

<figure><img src="image_10.png" /></figure>
<table><caption>**Table 1** Table summarizing the phases of the training step</caption><thead><tr><td>Phase</td><td>Description</td><td>Status</td></tr></thead><tbody><tr><td>Prepare</td><td>Create necessary folders and files for the training of the NNPs (and the number of NNPs to be trained)</td><td>Mandatory</td></tr><tr><td>Launch</td><td>Submit training jobs</td><td>Mandatory</td></tr><tr><td>Check</td><td>Check if the training jobs are successful</td><td>Mandatory</td></tr><tr><td>Freeze</td><td>Freeze the NNPs</td><td>Mandatory</td></tr><tr><td>Check_freeze</td><td>Check if the freezing is successful</td><td>Mandatory</td></tr><tr><td>Compress</td><td>Compress the NNPs</td><td>Optional</td></tr><tr><td>Check_compress</td><td>Check if the compression is successful</td><td>Optional</td></tr><tr><td>Increment</td><td>Update the temporary number</td><td>Mandatory</td></tr><tr><td>Clean</td><td>Remove unnecessary files</td><td>Optional</td></tr></tbody></table>

unnecessary training operations, enabling them to be efficiently used for inference (i.e., as NNPs predicting energies and forces), while the check_freeze phase will check the success of this operation. The compress phase will submit jobs to the HPC cluster to compress the models, and the check_compress phase will check the success of compression. The model compression⁹⁸ is used to boost the efficiency of inference using three techniques: tabulated inference, operator merging, and precise neighbor indexing. This is optional, and the user can choose to skip this phase. The final phase is the increment phase, which updates the iteration number, concluding the active learning cycle by producing a new generation of NNPs (or the first one). Fig. S3† shows a typical JSON output from this step, located in the control folder and named training_itera tionNUMBER.json, which records the results of each phase. After the training step is successfully completed, the user can proceed to the exploration step.

**3.3.3 Exploration.** This section details the exploration step and its goal: exploring the chemical space and selecting new candidates. This is done by propagating (biased) MD simulations with the current NNP generation, then performing a query-by-committee to select and extract inadequately described configurations.

The exploration step is at the core of the construction of a dataset using active learning in order to include representative structures potentially present along the reaction pathway(s). If the nuclei are treated classically, the current implementation calls LAMMPS for the exploration step, which is divided into the following phases: prepare, launch, check, deviate, extract, and clean. In the case of quantum nuclei, the exploration is performed using i-PI and is divided into the following phases:

prepare, launch, check, select_beads, rerun, deviate, extract, and clean. The overview of the phases is represented in Table 2.

The prepare phase creates the necessary folders and files to run MD simulations for each system using the concurrent NNPs trained at the previous step. The user can tune the number of trajectories to be run for each NNP (default value of 2). For example, for six systems, three NNPs, and two trajectories per NNP, a total of $n_{systems} \times n_{NNPs} \times n_{trajectories} = 6 \times 3 \times 2 = 36$ MD simulations will be prepared. Other tunable parameters include the timestep, the number of steps, and the machine keyword for the job scheduler parameters (see the complete list in the repository⁹³).

The launch phase will submit the MD simulations to the HPC cluster, and the check phase will ensure the success of the simulations. If some simulations have crashed, the user can choose to skip them, or to force the selection of candidates along the stable part of the trajectory. Indeed, it is very common in the early iterations for simulations to crash before the end when encountering structures deviating significantly from those on which they were trained. However, they can still be used to enrich the training database. During this phase, while the MD engine will propagate the trajectory using one of the NNPs, forces are also calculated on-the-fly with the other NNPs. For a given configuration $x$, the maximal deviation on the atomic forces, $\max_i[\epsilon_F(x)]$ is calculated as the maximal deviation on any atom $i$ within the configuration. The deviation of the atomic forces on atom $i$ for configuration $x$, calculated over $N$ NNPs, is defined as:

$$ \epsilon_{F,i}(x) = \sqrt{\frac{1}{N} \sum_{k=1}^{N} \|F_i(x, \text{NNP}_k) - \langle F_i(x, \text{NNP}_i) \rangle_{i=1...N}\|^2} \quad (1) $$

<table><caption>**Table 2** Table summarizing the phases of the exploration step, with the additional mandatory phases for PIMD exploration</caption><thead><tr><td>Phase</td><td>Description</td><td>Status</td></tr></thead><tbody><tr><td>Prepare</td><td>Create necessary folders and files for the exploration (per system)</td><td>Mandatory</td></tr><tr><td>Launch</td><td>Submit exploration jobs</td><td>Mandatory</td></tr><tr><td>Check</td><td>Check if the explorations are successful</td><td>Mandator</td></tr><tr><td>Select_beads</td><td>Select one random bead per configuration</td><td>Mandatory (PIMD only)</td></tr><tr><td>Rerun</td><td>Calculate the model deviation on those beads</td><td>Mandatory (PIMD only)</td></tr><tr><td>Deviate</td><td>Select new candidate configurations</td><td>Mandatory</td></tr><tr><td>Extract</td><td>Extract those configurations</td><td>Mandatory</td></tr><tr><td>Clean</td><td>Remove unnecessary files</td><td>Optional</td></tr></tbody></table>

<figure><img src="image_11.png" /></figure>
During the deviate phase, configurations are classified into three categories. Set A includes configurations that closely resemble parts of the training dataset and show minimal variance in the forces, $\max_i[\epsilon_{F,i}(x)] \le \sigma_{low}$. Set B includes configurations that present a significant variance in forces, $\sigma_{low} < \max_i[\epsilon_{F,i}(x)] \le \sigma_{high}$. Finally, set C includes configurations that are considered as potentially non-physical and unreliable with $\max_i[\epsilon_{F,i}(x)] > \sigma_{high}$. Configurations within set B will be referred to as candidates and will be labeled and added to the training dataset whereas configurations in set C will be discarded. The user can modify the values of $\sigma_{low}$ and $\sigma_{high}$, defining the range of set B.

We pause to discuss some useful guidelines for these values, which have been derived from our own experience. We recommend using a $\sigma_{low}$ of about four times the value of the NNP RMSE, which is typically around 0.05 eV Å⁻¹. Therefore, a value of 0.2 eV Å⁻¹ is a good starting point. Next, $\sigma_{high}$ can be set to four times this value, i.e., 0.8 eV Å⁻¹. At the later stage of the iterations, the user can reduce these values to 0.1 eV Å⁻¹ and 0.4 eV Å⁻¹ in order to limit the number of selected configurations once the dataset becomes rich enough in reactive structures. A third value, $\sigma_{max}$, acts as a threshold beyond which, even if configurations encountered afterwards during the dynamics drop below $\sigma_{high}$, they will still be discarded as the path to these configurations is deemed unphysical, with a default value of 1.0 eV Å⁻¹. The user can also set the maximum number of candidates to select, which is set to 50 by default (for each system), and also set how many timesteps are ignored at the beginning of each trajectory to ensure proper decorrelation from the starting point.

The deviate phase also selects starting points for the exploration step of the next iteration. These are chosen to be the configurations with the lowest deviation in set B, or, in the absence of such candidates, as the last configuration of the dynamics (which belongs to set A). If no new candidate emerges due to simulations crashing, the starting points of the explorations of the next iteration will be the same as those in the current iteration. This ensures that the next starting points are either part of the training dataset (because they will be candidates belonging to set B) or already well described by the NNPs (set A). Users also have the option to always start from the same initial configurations, which can be useful at the beginning of the iterative cycle.

The extract phase then extracts from the trajectories the selected starting points and candidates for the next step by reading the list of indices from the deviate phase. As the retrieval process can be time-consuming (on the order of minutes, especially if the trajectory files are large), the selection of candidates is split into two phases: the first (*vide supra*) is fast as only the deviation files are read, and the user can fine-tune the parameters (the $\sigma$ or the maximum number of candidates) and only then proceeds to the extract phase to process the trajectory files and retrieve the candidate configurations. Furthermore, users have the option to increase the number of candidates twofold by slightly shifting the positions of the atoms,⁶⁵ either for a specific set of atoms or for all atoms. This process applies to all original candidates, resulting in a final

number of candidates that includes both the original and the altered ones. This is done using the built-in function of Atomsk⁹² to disturb atomic positions by applying random translation vectors to atoms, while ensuring no global translation of the system and following a normal distribution function to generate new configurations. This can be useful when the exploration phase does not yield enough candidates or if the user wishes to explore a wider range of the phase space. Caution is emphasized, as the disturbed move is performed randomly and could lead to unphysical configurations, and can be also time-consuming if the number of candidates is large.

ArcaNN also offers the possibility to train the NNPs for nuclear quantum effects using RPMD and in that case, path-integral MD are run with i-PI. To have accurate NNPs to perform the RPMD simulations, they are trained on the beads and not on the centroids, as the NNPs will be used to compute forces on each bead. It is possible to use NNPs trained on PIMD simulations to perform classical MD simulations as the classical nuclei lie between beads; thus the NNPs can interpolate the computed forces (and energies), but the beads cannot be reliably extrapolated from training with classical nuclei, and thus caution is advised in the latter case. To achieve this, the exploration step has two new phases, select_beads and rerun. As i-PI does not allow multiple models to calculate the model deviation on-the-fly, the select_beads phase will randomly select one bead per MD step, and the rerun phase will run inference on the 'trajectory' to determine the deviation between the models using LAMMPS. The user can also mix classical and path-integral MD simulations, with one set of systems for each type of simulation.

It is important to note that most of the exploration (during the prepare phase) and selection (during the deviate phase) parameters, as well as the possibility to create new perturbed configurations (during the extract phase), are set independently for each system, providing great flexibility to the user, who can either use the same values for all systems or set different values for each system. Identical to the training step, a JSON output is written, and an example is shown in Fig. S4.†

A key point is that if the previous iteration $N$ results in a limited pool of candidates, ArcaNN dynamically adjusts the MD simulation lengths for the following exploration phase $N+1$, aiming to increase the sampling. After the exploration step is successfully completed, the user can proceed to the labeling step.

### 3.3.4 Labeling.
This section describes the labeling step. It will present the methods used to label the new candidates selected in the exploration step, which will then enrich the training dataset.

The goal of this step is to generate labels for the candidates selected in the exploration step, which will then enrich the training dataset. This step is divided into several phases: prepare, launch, check, extract, and clean with an overview of the phases presented in Table 3.

As with the other steps, the prepare phase will ensure the creation of necessary folders and files to run the single-point calculations. A few options are available to the user besides providing the input files for CP2K, namely the number of nodes,

<figure><img src="image_12.png" /></figure>
<figure><img src="image_13.png" /></figure>

<table><caption>**Table 3** Table summarizing the phases of the labeling step</caption><thead><tr><td>Phase</td><td>Description</td><td>Status</td></tr></thead><tbody><tr><td>Prepare</td><td>Create necessary folders and files for the labeling of the candidates</td><td>Mandatory</td></tr><tr><td>Launch</td><td>Submit the labeling jobs</td><td>Mandatory</td></tr><tr><td>Check</td><td>Check if the labeling jobs are successful</td><td>Mandatory</td></tr><tr><td>Extract</td><td>Extract the labeled candidates</td><td>Mandatory</td></tr><tr><td>Clean</td><td>Remove unnecessary files</td><td>Optional</td></tr></tbody></table>

the number of MPI processes per node, as well as the number of threads per MPI process. To improve efficiency, the single-point (SP) calculations are divided into two parts: the first SP calculation can be a quick and cheap calculation (e.g., GGA with a small basis set) to get an initial optimized wavefunction which will serve as a guess for the second SP calculation at the desired reference level of theory (e.g., GGA or hybrid-GGA with a large basis set). This significantly speeds up the labeling calculations.

The launch phase will submit the single-point calculations to the HPC cluster, and the check phase will ensure the success of the calculations (i.e., the convergence of the calculations). If a low-cost calculation does not converge, a warning is displayed; however, if the subsequent expensive calculation converges, the program continues. If the expensive calculation does not converge, an error is displayed, and the user will have to fix the issue before relaunching the phase, either by skipping the candidate or by manually relaunching the single-point calculations.

The extract phase will extract the molecular structure, energy, forces, box size, and, if present, the virial tensor from the single-point calculations and store them in the DeePMD-kit format as a new dataset. By convention, the files containing these new labeled structures are named sysname_XXX, where sysname is the name of the system and XXX is the iteration number.

As per the previous steps, a JSON file is written and is shown in Fig. S5.† After the labeling step is successfully completed, the user can proceed to the training step, completing the cycle.

**3.3.5 Test.** An optional step, the test step is used to test the performances of NNPs against the reference methodology after each training step. This step is divided into several phases: prepare, launch, check, clean, with an overview of the phases presented in Table 4.

The prepare phase will ensure the creation of the necessary folders and files to run the testing phase. It is important to note that here, the testing is performed on all datasets, including the initial, iterative, and extra datasets. This is not a validation of the NNPs, but a way to ensure that the NNPs are still performing

well on all datasets. For a more in-depth validation, the user should provide a separate dataset that was not used for training. The launch phase will submit the testing jobs to the HPC cluster while the check phase will ensure the success of the testing jobs and record the results in a control JSON file (Fig. S6†).

# 4 Application to typical chemical reactions

In this section, we demonstrate the use and capabilities of ArcaNN in training NNPs on two examples: a nucleophilic substitution reaction in solution and a pericyclic reaction in the gas phase. These two reactions are selected as model test cases for which all the necessary files are provided; however, we stress that a prototype version of ArcaNN has been successfully used for more complex sequential reactions involving nucleophilic attack, nucleofuge departure and proton rearrangements.¹⁴,¹⁵

## 4.1 Nucleophilic substitution reaction

We focus on the S<sub>N</sub>2 reaction between chloromethane CH<sub>3</sub>Cl and a bromide ion Br<sup>-</sup> in acetonitrile CH<sub>3</sub>CN, represented in Fig. 3. This reaction together with other related S<sub>N</sub>2 reactions has already been studied using a range of methods including mixed QM/MM simulations and *ab initio* molecular dynamics.<sup>99-106</sup>

The mechanism involves a single step wherein the Br<sup>-</sup> nucleophile attacks the chloromethane electrophilic carbon from the opposite side of the Cl leaving group. The nucleophilic attack and leaving group departure occur concurrently, leading to the inversion of the carbon center stereochemistry.

**4.1.1 Training of the NNPs.** We present here the key steps of our training strategy, and refer the reader to the Methods section and to the ESI† for technical details. All the input files, the labeled datasets, and the NNPs at each iteration are provided on the Github so that interested users can reproduce this procedure step by step.

<table><caption>**Table 4** Table summarizing the phases of the test step</caption><thead><tr><td>Phase</td><td>Description</td><td>Status</td></tr></thead><tbody><tr><td>Prepare</td><td>Create necessary folders and files for the testing of the NNPs</td><td>Mandatory</td></tr><tr><td>Launch</td><td>Submit the testing jobs</td><td>Mandatory</td></tr><tr><td>Check</td><td>Check if the testing jobs are successful and concatenate the results in a JSON file</td><td>Mandatory</td></tr><tr><td>Clean</td><td>Remove unnecessary files</td><td>Optional</td></tr></tbody></table>
<figure><img src="image_14.png" /></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 3 (A) Mechanism of the S<sub>N</sub>2 reaction between chloromethane and bromide ions. Collective variables used to bias or to monitor the reaction: (B) the distances between the carbon atom of the methyl group and the chlorine and bromine atoms, *d*<sub>C-Cl</sub> and *d*<sub>C-Br</sub>, respectively; (C) the angle α between the carbon atom of the methyl group and the chlorine and bromine atoms; (D) the angle ω between the plane containing the three hydrogen atoms of the methyl group (purple) and the plane containing the carbon atom of the methyl group and two hydrogen atoms (orange).</figcaption></figure>

4.1.1.1 *Initial aiMD dataset.* We start from an exploration of the system in the reactant state (i.e., CH<sub>3</sub>Cl + Br<sup>-</sup>) using a classical force field. From this trajectory 20 snapshots were extracted with half of them having their bromine and chlorine atoms swapped. Using these as starting points, very short aiMD trajectories were propagated at the DFT BLYP-D3 level. By extracting structures that are as decorrelated in time as possible, we generated an initial dataset of 1000 configurations, which will be referred to as the aiMD training dataset (see Methods and the ESI†).

4.1.1.2 *Iterative non-reactive datasets.* We first performed iterations of the exploration, labeling, and training steps (Fig. 2). The goal was to enrich the dataset with structures not well predicted by a given iteration of the NNP, while not explicitly training for reactivity yet. In practice, we generated a number of systems that allowed scanning the diversity of arrangements between the two molecules in the reactant and product states. After 7 such iterations, we decided to stop this procedure, as the number of new candidates to be included in the dataset became negligible. We refer to each generation *i* of datasets (and their corresponding NNPs) as *NR*<sub>*i*</sub> (for non-reactive). These steps resulted in a modest enrichment of the initial dataset, with a total number of 1158 structures in NR7.

4.1.1.3 *Exploration of reactive structures.* Finally, we performed 5 iterations of the exploration, labeling, and training steps with now explicit exploration of structures along the reaction pathway. This was achieved using a variety of systems based on 1D or 2D OPES. We refer to each generation *i* of datasets (and their corresponding NNPs) as *Ri* (for non-reactive). These steps resulted in a significant enrichment of the initial dataset, with a total number of 2313 (1000 + 158 + 1155 structures) structures in R5. Although some OPES trajectories crashed during the exploration of reactive structures with intermediate datasets, simulations with R5 were found to be stable and we thus decided to stop the dataset construction and training after 5 steps (Fig. S7†).

**4.1.2 Validation of the datasets and their corresponding NNPs.** In this section, we will present how the validation of the

training datasets was done and show the advantage of using ArcaNN. We will detail how to assess the quality of the training, which is essential to ensure the reliability of the NNPs in the case of a chemical reaction, using different metrics.

We now discuss the benefits of the ArcaNN approach by comparing a variety of observables along the iterations. For this purpose, we first constructed a test dataset that is relevant for the chemical reaction by systematically generating 1210 structures along the reactive path between the reactant and product basins using Umbrella Sampling (US) simulations with the final R5 NNP (see Methods). Having a test dataset is critical to assess the quality of the training,<sup>107,108</sup> and it is generally uniformly sampled along the entire phase space. To study a particular reaction, we believe that a test dataset of untrained structures uniformly sampled along the reaction pathway permits ensuring that the accuracy of the NNPs is constant for all relevant reactive structures. This is even more important if the reaction presents two pathways: both should be described with the same accuracy. Irrespective of the ArcaNN procedure, we also performed two types of enhanced sampling “production-like” simulations at each cycle with the resulting NNPs: US and OPES simulations. We tracked the occurrence of untrustworthy structures in the US simulations and, for both methods, the free-energy surfaces for the reaction. These are the metrics we used to determine the validity of the NNPs: the RMSE of the forces for an independent test set along the reaction pathway to ensure accuracy and the stability of the NNPs during enhanced sampling.

Fig. 4A shows the Root Mean Square Error (RMSE) of the force components in the training and test datasets at each ArcaNN cycle. Until the final iteration of the non-reactive dataset, we do not observe significant variations in the RMSE on the training datasets, suggesting that the NNPs train with similar accuracy, which is not surprising considering the limited augmentation of the training dataset during these iterations. However, these steps are essential to start mapping the chemical phase space, as the initial aiMD dataset contains a very inhomogeneous distribution of structures, with, for example, very few reactant configurations where CH<sub>3</sub>Cl and Br<sup>-</sup> are far apart (Fig. 4B).

We notice a clear gap between the RMSE on the training dataset and that on the test dataset that encompasses a lot of reactive structures on which these non-reactive NNPs have not been trained. However, even without the explicit inclusion of structures on the reaction pathway, the NNPs get better at extrapolating the corresponding forces, leading to a small but noticeable decrease in the RMSE on the test dataset.

When we start reactive cycles, the RMSE on the training dataset suddenly increases, while the error on the testing dataset decreases. This can be explained by the large number of new structures that are included in the dataset during the reactive cycles, especially close the transition state region (Fig. 4B). As the training dataset increases in size and diversity, overfitting on the initial structures is reduced, while simultaneously improving the description of the newly encountered higher energy configurations near the transition state, which
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 4 ArcaNN training for the S<sub>N</sub>2 reaction (A) histogram of the RMSE of the forces on the training dataset and the test dataset at each training cycle, with aiMD corresponding to the initial dataset, NR for each non-reactive ArcaNN cycle, and R for each reactive ArcaNN cycle. (B) Histogram of the percentage of all untrustworthy structures from the US calculations (where max<sub>i</sub>|e<sub>F,i</sub>(x)| > 0.7 eV Å<sup>-1</sup>) with the NNPs obtained at each ArcaNN cycle. (C) Probability density of untrustworthy structures in the training datasets as a function of the absolute value of δd, with aiMD representing the initial structures, NR7 representing all 158 structures added during the non-reactive ArcaNN cycles, and R5 representing all 1155 structures added during the reactive ArcaNN cycles. (D) Free energy profile of the US calculations with the NNPs obtained at each ArcaNN cycle.</figcaption></figure>

were responsible of the poor initial performances on the testing dataset.

The only observation of the RSMEs can lead to deceptive conclusions about the necessity of iterations and the progressive exploration of the chemical phase space. Therefore, this should not be the sole aspect to consider when assessing the convergence and the quality of the NNPs for a given chemical reaction. For example, for each generation of NNPs, we report in Fig. 4C the fraction of structures encountered during 1D US simulations (such as those presented in Fig. 4D) that result in large deviations from the reference method. While the original aiMD NNPs appeared to yield reasonable RMSEs (Fig. 4A), they result in a significant fraction of poor predictions along the reaction pathway. During the non-reactive cycles, the NNPs get progressively better, with NR2 and NR3 that seem to be reliable. However, this further degrades again when continuing the non-reactive iterations, which seems surprising since the global RMSE on the test dataset keeps decreasing, although to a limited extent. This suggests that the non-reactive cycles here could probably have been stopped after the third iteration.

When starting the reactive cycles, the NNPs become more and more reliable when considering the fraction of untrustworthy structures (Fig. 4C and S7†), which goes to zero for the fifth iteration R5. However, things do not seem to significantly improve after R2. In Fig. S8,† we represent the RMSEs along the reaction coordinate for the aiMD, the NR7 and the R5 NNPs: one

can see that at the final iteration, the RMSEs is constant for all structures encountered along the reaction pathway. The RMSEs for the R5 NNPs and the test dataset are reported in Fig. S9.† The RMSEs on the magnitude of the forces are similar for the training and test datasets with a value around 0.05 eV Å⁻¹, whereas the RMSEs on the force components are equal and slightly lower, with values around 0.03 eV Å⁻¹.

One key aspect that is overlooked in these considerations is the stability of the NNPs when running the actual simulations, especially so when using enhanced sampling methods. For example, when running the 1D US simulations for each generation of NNPs, many windows crash after a few tens to a few hundreds of ps. This is observed for all NNPs except the last one (R5). However, these simulations provide enough data to allow for overlap between adjacent windows along this collective variable, and the corresponding PMFs can be determined (Fig. 4D). Despite being unstable, the intermediate NNPs lead to free-energy profiles that do not exhibit major inconsistencies, although the barrier appears to be not quantitatively described when the aiMD or non-reactive NNPs are used. Strikingly, the transition state (TS) structure is not correct, being a carbocation, as in a S<sub>N</sub>1 mechanism (Fig. S10†). For more complex reactions involving several atom exchanges (for example, proton transfers in addition to a heavy atom exchange), it is expected that free-energy surfaces would not easily converge.
However, US simulations give seemingly physical results with non reactive NNPs for this specific case, which may fool the user into believing that subsequent optimization of the NNPs are not required. As already mentioned, the fact that all but the final R5 NNPs result in at least one non stable trajectory is already an indication that they should not fully be reliable. Long enhanced sampling simulations using, e.g., OPES appear as a more stringent test of the quality of these NNPs (see Methods for details).

For example, OPES simulations with NNPs from the R1 dataset crash after 44 ps while the one with the R3 dataset does not crash but starts becoming untrustworthy after 978.25 ps. When accounting for the bias accumulated until they crash or become untrustworthy, we can reconstruct free energy surfaces along the carbon-halogen distances (see Fig. 5), which are not correct at all and exhibit unrealistic basins. Only the final R5 NNP converges to a $\Delta G^{\ddagger}$ equal to $14.74 \pm 0.39 \text{ kcal mol}^{-1}$ and

a $\Delta G$ equal to $2.25 \pm 0.44 \text{ kcal mol}^{-1}$, similar to the values obtained from the US simulations with the same NNP (see below).

These results illustrate that the RMSE of the forces on a test dataset is not enough to ensure the validity of the NNPs. One must also check the stability of the NNPs during enhanced sampling simulations, because the explored pathways are not always the minimum free energy paths and US simulations with very high number of windows and good overlap can mask this instability. We recommend to use several types of enhanced sampling simulations to ensure the stability of the NNPs, ideally using a superset of those that will be used for the study of the reaction, especially when the reaction requires more than one collective variable for description.

### 4.1.3 Thermodynamics and mechanism of the model reaction.
We now present how the final NNPs can be used to study the S<sub>N</sub>2 reaction between chloromethane and bromide ions in acetonitrile. This will be done using two types of enhanced sampling simulations: Umbrella Sampling (US) and On-the-fly Probability Enhanced Sampling (OPES).

We eventually illustrate how the final, stable R5 NNP can lead to quantitative and accurate information about this model S<sub>N</sub>2 reaction. In Fig. 6A, we show the free-energy profile along the asymmetric stretch of the carbon-halogen distances $\delta d$, together with the evolution of these distances and of the $\omega$ angle, indicating the Walden inversion. Fig. 6B–D show some joint probabilities of these key collective variables (CVs) throughout the reaction.

Based on the free-energy profile, we determined the reaction free energy, directly from the free energy profile, $\Delta G$ to be $2.20 \pm 0.23 \text{ kcal mol}^{-1}$ and the reaction free energy barrier $\Delta G^{\ddagger}$ to be $14.46 \pm 0.17 \text{ kcal mol}^{-1}$. The transition state is located at $\delta d = -0.175 \AA$, consistent with an S<sub>N</sub>2 reaction and an associative mechanism as we can see in Fig. 6A. At $\delta d = -0.175 \AA$, the distances $d_{C-Cl}$ and $d_{C-Br}$ are equal to $2.4 \AA$ and $2.575 \AA$, respectively.

In Fig. 6B, the density distribution of the cosine of the $\alpha$ angle, formed by the chlorine atom, the carbon atom, and the bromine atom (see Fig. 3C), along $\delta d$, is reported. In both the reactant and product states, $\alpha$ is uniformly distributed at large distances when the two molecules do not interact, but becomes more and more colinear as we approach the transition state, taking a value of 171°. This behavior is expected for the S<sub>N</sub>2 reaction mechanism, where the nucleophile attacks the carbon atom from the opposite side of the leaving group. The density distribution of the $\omega$ angle, defined as the angle between the plane formed by the three hydrogens of the chloromethane and the plane formed by the carbon and two of the three hydrogens of the chloromethane, is reported in Fig. 6C. $\omega$ (see Fig. 3D) takes a value of 32.6° in the reactant state and –30.9° in the product state, reaching a value of 2.3° at the transition state, demonstrating a Walden inversion¹⁰⁹ of chloromethane, characteristic of the S<sub>N</sub>2 reaction.

## 4.2 Diels-Alder reaction

We now illustrate the capabilities of ArcaNN on another type of reaction. We select a pericyclic reaction consisting of a [4 + 2]

<figure><img src="image_18.png" /><figcaption>Fig. 5 Free energy surfaces for the S<sub>N</sub>2 reaction obtained from OPES simulations with the NNPs trained, respectively, on the R1 (A), R3 (B) and R5 (C) datasets.</figcaption></figure>

<figure><img src="image_19.png" /></figure>
<figure><img src="image_20.png" /></figure>

<figure><img src="image_21.png" /><figcaption>Fig. 6 (A) Free energy surface of the S<sub>N2</sub> reaction obtained from the US simulations with the NNP trained on the R5 dataset, with the average value (solid colors) and the 95% confidence interval (shaded colors) and the average value of the collective variables (as well as the 95% confidence interval) for each block of the US simulations (shaded colors). (B) Joint density distribution of the distance *d*<sub>C–Cl</sub> and the distance *d*<sub>C–Br</sub>, with the dotted line representing *δd* = 0 Å obtained from the US simulations. (C) Joint density distribution of the ω angle and *δd* obtained from the US simulations. (D) Joint density distribution of the cosine of the α angle and *δd* obtained from the US simulations.</figcaption></figure>

addition: the Diels–Alder reaction between ethylene (C₂H₄) and 1,3-butadiene (C₄H₆) in the gas phase, forming cyclohexene (C₆H₁₀) (see Fig. 7A). This reaction has been extensively studied using a wide range of theoretical methods.<sup>12,110–113</sup> For simplicity, we focus on the reactivity of the *s*-cis conformation of 1,3-butadiene, which is the most reactive form of the molecule<sup>112</sup>

**4.2.1 Training of the NNPs.** In the following, we briefly describe the key steps involved in the training of the NNPs. Extensive technical details are given in the Methods section and in the ESI.† Input files and labeled datasets are provided in the Github repository.

The absence of explicit solvent molecules drastically reduces the number of degrees of freedom and hence the training computational complexity. As a consequence, we directly initiated our training with short aiMD simulations sampling the transition between the reactant and product, propagated at the BLYP-D3 DFT level. We performed one simulation in the reactant state (C₂H₄ + C₄H₆), with the two molecules kept at close distance, one in the product state (C₆H₁₀), and two steered-MD simulations along *d̄*, respectively from the reactant to the product and from the product to the reactant. From these four trajectories, we generated the aiMD training dataset consisting of 244 structures (61 structures per trajectory) (see Methods and the ESI†).

From this initial dataset, we started the ArcaNN procedure with a mixture of non-reactive and reactive systems based on

steered-MD and 1D OPES along the average *d̄* of the two distances (*d*₁ and *d*₂), corresponding to the newly formed bonds, see Fig. 7B. We performed 8 iterations of the exploration, labeling, and training steps, with a total number of 3519 (244 + 3275) structures in the final dataset, named R8. The training was considered as converged at this point as very few new structures were added to the dataset during the last iteration (<1% of the total number of structures generated during the last exploration).

To assess the quality of the NNPs, we constructed a test dataset of 1095 structures along *d̄* generated using US simulations with the final R8 NNP (which were primarily used to calculate the reaction free-energy landscape, see Methods). In Fig. 7C, we show the distribution of errors on the force components in the training and test datasets at the final (R8) cycle. The RMSE of the component of the forces is ≈0.07 eV Å⁻¹ on the training dataset and ≈0.07 eV Å⁻¹ on the test set (see Fig. S11†); the RMSE on the forces along the reaction pathway is represented in Fig. S12.†

**4.2.2 Thermodynamics and mechanism of the model reaction.** Using the final R8 NNP, we performed US simulations along the collective variable *d̄* to calculate the free energy profile of the Diels–Alder reaction (Fig. 7D). The minimum at *d̄* = 1.5 Å corresponds to the product state (C₆H₁₀), the relatively flat region beyond *d̄* = 4.85 Å corresponds to the reactant state (C₄H₆ + C₂H₄), and the maximum at *d̄* = 2.25 Å corresponds to the transition state. The ΔG and ΔG‡ were calculated from the
<figure><img src="image_22.png" /><figcaption>Fig. 7 (A) Mechanism of the Diels–Alder reaction between ethene and s-cis-1,3-butadiene. (B) Key collective variables used to describe the reaction: the distances $d_1$ and $d_2$ (C) Probability density of the component-wise force errors in the training and test datasets. (D) Free energy profile along the average $\bar{d}$ of the $d_1$ and $d_2$ distances obtained from US simulations with the NNP trained on the R8 dataset, with the average value (solid colors) and the 95% confidence interval (shaded colors).</figcaption></figure>

free energy profile to be −19.0 ± 0.2 kcal mol⁻¹ and 28.1 ± 0.1 kcal mol⁻¹, respectively. This is in fair agreement with the work of Cui and Liu,¹¹² who reported values for ΔG of −14.3 kcal mol⁻¹ and ΔG‡ of 33.2 kcal mol⁻¹ using a static approach at the same level of theory. As per the S<sub>N</sub>2 reaction, we just report the free energy difference between the different states. We note that our reactant state is not at infinite distance as in the work of Cui and Liu. If we examine the more precisely defined ΔG‡ of the reverse process, i.e., the ring-opening reaction, we find an excellent agreement between our computed value 47.1 ± 0.2 kcal mol⁻¹ and the previously-published value of 47.5 kcal mol⁻¹.

For this prototypical Diels-Alder reaction, our simulations suggest that the mechanism is concerted and quasi-synchronous, with the two bonds forming at the same time, in agreement with the literature.¹¹⁰,¹¹⁴,¹¹⁵ This can be seen on the probability density distribution of the $d_1$ and $d_2$ distances along the reaction coordinate $\bar{d}$ (see Fig. S13†).

## 4.3 Methods

The Methods section outlines the generation of initial datasets from aiMD simulations and the subsequent training of NNPs with ArcaNN. It details the non-reactive and reactive iterative training cycles, including dataset augmentation and parameter settings. Finally, it describes the production simulations performed using US (and OPES for the S<sub>N</sub>2) simulations to explore system reactivity and calculate free energy profiles.

### 4.3.1 Initial datasets.
The initial datasets were generated through *ab initio* molecular dynamics (aiMD) simulations for both the S<sub>N</sub>2 and Diels-Alder reactions. For the S<sub>N</sub>2 reaction, twenty trajectories of 2 ps each were performed with a timestep of 0.5 fs. Ten trajectories started from the reactant state (CH<sub>3</sub>Cl + Br<sup>-</sup>) and the other ten from the product state (CH<sub>3</sub>Br + Cl<sup>-</sup>). In

the Diels-Alder simulations, four trajectories of the same length and timestep were conducted: one initiated from the reactant state (C<sub>4</sub>H<sub>6</sub> + C<sub>2</sub>H<sub>4</sub>) with the molecules in close proximity, another from the product state (C<sub>6</sub>H<sub>10</sub>), and two steered-MD simulations transitioning between reactant and product states in both directions. Structures were extracted every 30 fs from each aiMD trajectory after discarding the initial 0.5 fs to ensure proper decorrelation. For the S<sub>N</sub>2 reaction, these structures were combined into two sets – one for the reactant and one for the product – each containing 500 configurations. In the case of the Diels-Alder reaction, the structures were grouped into four sets corresponding to each trajectory, totaling 244 configurations. All configurations were labeled at the BLYP-D3 level of theory using the TZV2P-MOLOPT basis set for S<sub>N</sub>2 and the TZV2P basis set for the Diels-Alder reaction, along with GTH pseudopotentials for both; this is referred to as the reference level. The molecular structures, along with their corresponding box sizes, energies, forces, and virial tensors, were extracted and stored in the DeePMD-kit format. These datasets were then provided as initial inputs for use with ArcaNN, comprising 1000 configurations divided into two datasets for the S<sub>N</sub>2 reaction and 244 configurations divided into four datasets for the Diels-Alder reaction, collectively referred to as the aiMD training dataset.

### 4.3.2 Initialization.
In the S<sub>N</sub>2 case, this step uses 6 systems with 3 starting from the reactant state and 3 from the product state. For the reactant state systems, one system had no restraint, one had a flat-bottom restraint on the distance between the carbon atom of the methyl group and the bromide ion ($d_{C-Br} \le 3.0$ Å, with a force constant $\kappa = 5.0$ kcal mol<sup>-1</sup> Å<sup>-2</sup>), and the last one with a moving harmonic bias (steered-MD) on the $d_{C-Br}$ distance ranging from 2.5 Å to 10.0 Å with a force constant of 1.0 kcal mol<sup>-1</sup> Å<sup>-2</sup>. For the product state systems,
the same three systems were used, but with the $d_{C-Cl}$ distance. For the Diels-Alder reaction, 10 systems were used: two in the reactant and product states without any enhanced sampling; two with steered-MD transitioning from the reactant to the product and *vice versa*, acting on both $d_1$ and $d_2$ distances from 3.5 Å (1.5 Å) to 1.5 Å (3.5 Å) over 10 ps with a force constant of 100 kcal mol$^{-1}$ Å$^{-2}$; and six using OPES acting on the collective variable $\bar{d}$, with initial $\sigma = 0.05$ Å, a deposition pace of 500 timesteps, and $\Delta E$ values of 20 kcal mol$^{-1}$, 50 kcal mol$^{-1}$, and 70 kcal mol$^{-1}$, starting from both reactant and product states. In both cases, all ArcaNN parameters were maintained at their default values; 3 NNPs were trained for the committee with 2 trajectories per NNP used during the exploration step.

**4.3.3 Training.** The training was performed with DeePMDKit$^{87}$ version 2.1, with an initial learning rate of 0.001 and a final learning rate of $10^{-6}$, a decay rate of 0.92, decay steps of 5000, and a total of 400 000 steps. The DeepPot-SE scheme was utilized, setting the cutoff for radial and angular information at 6 Å and applying a cosine weight function for atoms located beyond 0.5 Å. The embedding neural network that maps the environment matrix to a local embedding matrix contains 3 hidden layers with 25, 50, and 100 nodes, respectively. The following fitting neural network that maps the descriptor to the atomic energy contains 3 hidden layers with 240 nodes each. The initial and final energy loss prefactors were set to 0.01 and 1, respectively, and the force loss prefactors were set to 1000 and 1, respectively.

**4.3.4 S<sub>N2</sub> non reactive exploration.** The initial exploration was performed using LAMMPS, with a timestep of 0.5 fs, a total of 20 000 steps, and a print interval of 200 MD steps (i.e., 1% of the total length). The simulations were conducted in the NVT ensemble at 300 K with a CSVR thermostat$^{116}$ and a time constant of 0.1 ps. The maximum deviation on the atomic forces was set to 0.15 for $\sigma_{low}$, 0.7 for $\sigma_{high}$, and 1.0 eV Å$^{-1}$ for $\sigma_{max}$ as the candidate selection criteria. At the seventh iteration, only 5 candidates were selected from the 36 MD simulations (three NNPs, two per NNP, and six systems), each lasting 400 ps. Therefore, it was decided to restart the ArcaNN procedure with a biased exploration to include reactive structures. The total number of configurations in the training dataset at this point was 1158, which will be referred to as the NR7 training dataset.

**4.3.5 S<sub>N2</sub> reactive exploration.** The ArcaNN procedure was restarted with an augmented dataset containing the initial 1000 aiMD configurations plus the 158 configurations generated by the seven non-reactive cycles. For this new biased iterative training, twelve systems were created, each with a different starting configuration for the exploration step. Six systems were used to explore the reactivity using OPES from the reactant state, with three systems where the CV was the $\delta d = d_{C-Br} - d_{C-Cl}$ reaction coordinate and OPES parameters were set to $\sigma = 0.05$ Å, a deposition pace of 2000 timesteps, and $\Delta E$ values of 5 kcal·mol$^{-1}$, 10 kcal·mol$^{-1}$, and 20 kcal·mol$^{-1}$. For the other three systems, bias was applied to the $d_{C-Br}$ and $d_{C-Cl}$ distances, with initial values of $\sigma = 0.05$ Å for both, a deposition pace of 2000 timesteps, and $\Delta E$ equal to 5 kcal·mol$^{-1}$, 10 kcal·mol$^{-1}$, and 20 kcal·mol$^{-1}$. The same parameters were used for the 6 systems exploring the reactivity using OPES from the product state (with

3 OPES 1D and 3 OPES 2D). A total of 1155 new configurations from these biased explorations were added to the training dataset. After 7 non-reactive cycles and 5 reactive cycles, the number of configurations in the training dataset was 2313, and a final training of the NNPs was performed on this R5 dataset. In Fig. 4A, we report the cumulative probability density of structures in the training datasets as a function of the reaction coordinate $\delta d$ for the aiMD dataset (1000 structures), the non-reactive dataset NR7 (1000 + 158 structures), and the reactive dataset R5 (1000 + 158 + 1155 structures). It can be seen that the transition region is well sampled with only with the addition of the reactive ArcaNN cycles, and that the non-reactive cycles are not enough to sample the transition region (see also Fig. S14†).

**4.3.6 Diels-Alder reactive exploration.** Using the 10 systems described above, the initial reactive exploration was performed using LAMMPS, with a timestep of 0.25 fs, a total of 20 000 steps, and a print interval of 200 MD steps (i.e., 1% of the total length), at 300 K with a CSVR thermostat$^{116}$ and a time constant of 0.1 ps$^{-1}$. After 8 iterations of the ArcaNN procedure, the final dataset contained 3519 configurations, referred to as the R8 training dataset.

**4.3.7 S<sub>N2</sub> production simulations.** Once the iterative training procedure was finished, the reactivity of the system was explored by performing US simulations with the final NNP (i.e., R5). The reaction coordinate was defined as the difference between the distance $d_{C-Cl}$ and the distance $d_{C-Br}$, $\delta d$ (see Fig. 3B). The reaction was divided into 121 windows, linearly spaced from $\delta d = -3.0$ Å to $\delta d = 3.0$ Å. All simulations thereafter were performed in the NVT ensemble at 300 K with a timestep of 0.5 fs using a CSVR thermostat$^{116}$ with a time constant of 0.1 ps$^{-1}$. For each window, the system was brought to an equilibrium state by performing steered-MD to the target value of $\delta d$, linearly over 50 ps with a spring constant of 200 kcal mol$^{-1}$ Å$^{-2}$. Then it was further equilibrated for 50 ps at the target value and production runs were carried out for 600 ps for each window. The total accrued simulation time was 50 ns and the simulation speed was roughly 6 ns per day on a single GPU. A test dataset was also generated by taking 10 random structures from each window of the production US simulations totalling 1210 structures along the reaction coordinate $\delta d$ and labeling them at the reference level of theory.

The 600 ps long production runs were divided into 6 blocks of 100 ps each, and the 1D free energy profile was calculated for each block using the Weighted Histogram Analysis Method (WHAM)$^{117}$ with 312 bins along $\delta d$. Then using each block result, the average and the 95% confidence interval were calculated by setting the free energy at 0 kcal mol$^{-1}$ at $\delta d = -1.95$. The $\Delta G$ and $\Delta G^\ddagger$ were calculated from the averaged 1D free energy profile as the difference between the free energy of the reactant (CH$_3$Cl + Br$^-$) and product states (CH$_3$Br + Cl$^-$) and the difference between the free energy of the reactant and the maximum (the transition state) of the free energy profile, respectively. For the collective variables, each structure for all windows (and the full duration) was binned to a grid of $\delta d$ values (same binning as the WHAM procedure), and the average and 95% confidence interval were calculated for each bin for the $d_{C-Cl}$ distance, the $d_{C-Br}$ distance, and the $\omega$ angle.

<figure><img src="image_23.png" /></figure>
<figure><img src="image_24.png" /></figure>

For the OPES simulations with the final R5 NNP, bias was applied to the $d_{C-Br}$ and $d_{C-Cl}$ distances, with $\sigma = 0.05$ Å for both, a deposition pace of 500 timesteps, and $\Delta E$ equal to 20 kcal·mol⁻¹. The simulation was propagated for 2.5 ns in the NVT ensemble at 300 K with a timestep of 0.5 fs using a CSVR thermostat¹¹⁶ with a time constant of 0.1 ps⁻¹ (same as the production US simulations). The 2D free energy surface was calculated by reweighting the biased simulations along the $d_{C-Br}$ and $d_{C-Cl}$ distances (Fig. 5). The simulations were divided into 5 blocks of 500 ps each, and the free energy was calculated for each block by reweighting along the $\delta d$ collective variables, permitting the calculation of an average and 95% interval 1D free energy profile (see Fig. S15†). The $\Delta G$ and $\Delta G^\ddagger$ were calculated as described above for the US simulations. The same procedure as the US simulations was used to calculate the average and 95% confidence interval for the $d_{C-Cl}$ distance, the $d_{C-Br}$ distance, and the $\omega$ angle.

### 4.3.8 Diels-Alder production simulations.
After completing the iterative training procedure, the system's reactivity was explored using US simulations with the final NNP, denoted as R8. The reaction coordinate was defined as the average of the two distances $d_1$ and $d_2$, $\bar{d}$ (see Fig. 7B). The reaction was divided into 73 windows, linearly spaced from $\bar{d} = 1.4$ Å to $\bar{d} = 5.0$ Å. All subsequent simulations were performed in the NVT ensemble at 300 K with a timestep of 0.5 fs, using a CSVR thermostat¹¹⁶ with a time constant of 0.1 ps⁻¹. To maintain the system in the s-cis conformation, a flat-bottom restraint was applied to the dihedral angle with a force constant of 100 kcal mol⁻¹ rad⁻² to keep it between -π/2 rad and π/2 rad. For each window, the system was equilibrated by performing steered-MD to the target $\bar{d}$ value over 50 ps with a spring constant of 1000 kcal mol⁻¹ Å⁻². This was followed by an additional 50 ps equilibration at the target value, and production runs of 600 ps for each window. The total simulation time accrued was 42.6 ns, with a simulation speed of approximately 24 ns per day on a single GPU. The free energy profile, along with its average and 95% confidence interval, as well as the $\Delta G$ and $\Delta G^\ddagger$ values, was calculated using the same procedure as for the $S_{N2}$ reaction.

### 4.3.9 $S_N2$ test simulations.
At each ArcaNN cycle, US simulations were performed with a similar protocol to the production of the final NNP. Using $\delta d$ as the reaction coordinate, 121 windows were used, with each starting point being the last geometry of the corresponding window on the production US simulations. The simulations were performed in the NVT ensemble at 300 K with a timestep of 0.5 fs using a CSVR thermostat¹¹⁶ with a time constant of 0.1 ps⁻¹ for 200 ps. The spring constant for each harmonic restraint was set to 200 kcal mol⁻¹ Å⁻² as per the US simulations. The free energy profile was then calculated using WHAM.¹¹⁷ For the OPES simulations done with the R1 and R3 NNP, exactly the same starting point and parameters were used as the R5 production one.

### 4.3.10 Diels-Alder test simulations.
A test dataset was generated by selecting 15 random structures from each window of the production US simulations, totaling 1095 structures

along the reaction coordinate $\bar{d}$, and labeling them at the reference level of theory.

## 5 Conclusion

ArcaNN addresses the challenge of generating training datasets for reactive MLIPs. By combining concurrent learning with advanced sampling techniques, ArcaNN facilitates the exploration of chemically relevant configurations, including high-energy geometries, and integrates classical and quantum nuclear dynamics into a standardized automated workflow. The framework is designed to be user-friendly and flexible, allowing researchers to easily set up and run ArcaNN to train neural network potentials (NNPs) for their reactive systems. We illustrated the power of ArcaNN in the context of two different reactions: first, a nucleophilic substitution ($S_{N2}$) reaction in explicit solvent, and second, a pericyclic reaction in the gas phase. In both cases, we demonstrated its capabilities in generating accurate and stable NNPs, both in the reactant and product region, but most importantly along the reaction pathway. Beyond these simple examples, preliminary versions of the code were used by us for much more complex reactions involving several molecular steps and multiple pathways.¹⁴,¹⁵ We also note that the training set obtained for these reactions can be used as a starting point to study similar but more complex reactions presenting different nucleophiles and leaving groups for the $S_{N2}$, or functionalized dienes and dienophiles for the Diels-Alder reaction, respectively. Although an initial aiMD training set is still needed to ensure a stable initial representation of the functionalized reagents (especially in the presence of new chemical elements), the reactive configurations in the present dataset will significantly accelerate the iterative procedure needed to refine the description of related systems exhibiting similar reactivity. We also provide guidelines on how to assess the quality of a NNP for a reactive system, suggesting that many aspects should be considered beyond the canonical RMSE on the energies and forces. Future developments of ArcaNN will include the incorporation of additional selection techniques, expansion to use other MLIPs, integration with different molecular dynamics engines, and support for various quantum chemistry packages for labeling.

Through continuous improvements, ArcaNN aims to facilitate the broader adoption and application of MLIPs in computational chemistry, enabling new advancements in chemical reactivity and catalysis.

## Data availability

The code for ArcaNN can be found at https://github.com/arcann-chem/arcann-training. An in-depth documentation is available at https://arcann-chem.github.io/arcann-training. The version of the code employed for this study is version 1. Necessary user files and initial aiMD datasets to start the training of the NNPs for the $S_{N2}$ and the Diels-Alder reaction with ArcaNN are available in the Examples section of the GitHub repository: https://github.com/arcann-chem/arcann-training.
## Author contributions

Rolf David: conceptualization, methodology, software, supervision, validation, formal analysis, investigation, visualization, writing-original draft, writing-review and editing. Miguel de la Puente: software, validation, resources, writing-review and editing. Axel Gomez: software, validation, resources, writing-review and editing. Olaia Anton: validation, resources, writing-original draft, writing-review and editing. Guillaume Stirnemann: conceptualization, funding acquisition, project administration, supervision, writing-original draft, writing-review and editing. Damien Laage: conceptualization, funding acquisition, project administration, supervision, writing-original draft, writing-review and editing.

## Conflicts of interest

There are no conflicts to declare.

## Acknowledgements

This work was supported by a postdoctoral fellowship to R. D. from PSL OCAV (Idex ANR-10-IDEX-0001-02PSL) and from the European Research Council (ERC) under the European Union's Eighth Framework Program (H2020/2014-2020)/ERC grant agreement no. 757111. This project was provided with HPC and storage resources by GENCI at IDRIS/TGCC, thanks to grants A0110707156, A0130707156, A0150707156, A0130811005, and A0150811005, on the Jean Zay and Joliot Curie supercomputers, specifically on the ROME, CSL, V100, and A100 partitions. The authors thank the ENS group members and collaborators who have tested the preliminary versions of the code: Zakarya Benayad, Pierre Vieillard, Oscar Gayraud, Pierre Girard, Anne Milet, Meritxell Malagarriga Pérez, Adrián García-Martínez, Ashley Borkowski, Pauf Neupane, Ward H. Thompson, Mohammadhasan Dinpajooh, and Tomonori Hirano.

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