MolMod, a web-based database for classical force fields for molecular simulations of fluids [Mol. Sim. 45, 10 (2019) 806-814], was extended to transferable force fields. Eight transferable force fields, including all-atom and united-atom type force fields, were implemented in the MolMod database: OPLS-UA, OPLS-AA, COMPASS, CHARMM, GROMOS, TraPPE, Potoff, and TAMie. These transferable force fields cover a large variety of chemical substance classes. The system is designed such that new transferable force fields can be readily integrated. A graphical user interface was implemented that enables constructing molecules. The MolMod database compiles the force field for the specified component and force field type and provides the cor- responding data and meta data as well as ready-to-use input files for the molecule for different simulation engines. This helps the user to flexibly choose molecular models and integrate them swiftly in their individual workflows – reducing risks of input errors in molecular simulations. 1 Introduction Molecular simulations with force fields have become an important tool for predicting ther- mophysical properties1–10as well as for studying nanoscopic processes11–18in many scientific disciplines. The link of the simulations to the real world is established by the employed substance models, i.e. by the force fields. They provide mathematical expressions describ- ing the interactions between the particles on the molecular level and are usually formulated as interaction potentials between different interaction sites. Therein, the interaction sites describe atoms or groups of atoms. The choice of the force field is decisive for the reliabil- ity of the predictions obtained from the molecular simulation.19–22A large number of force fields has been proposed in the past decades. Yet, their electronic accessibility and findabil- ity23,24is still cumbersome. Despite the abundant importance of force fields for molecular simulations, data science principles such as FAIR23(findable, accessible, interoperable, and reusable) have received limited attention by the molecular simulation community. Therefore, there is a lack of data standards and electronically available models. This also affects the reproducibility of molecular simulation computer experiments, which has been identified as a problem in recent years.25–30The lack of force field databases for finding and reusing force field models has been stressed in the past.24 Two types of force fields can be distinguished: (i) component-specific force fields and (ii) transferable force fields. Component-specific force fields are substance models that were individually designed and parameterized for modeling a given substance. Hence, component- specific force fields describe solely the molecular interactions of a particular chemical sub- stance. The generalization of a component-specific force field to chemically related or similar substances is in general not feasible. Transferable force fields31–34on the other hand are generalized chemical construction plans that can be used for building a large number of substance models based on transferable building blocks. Hence, the interaction parameters of the sites of a transferable force field can be applied to a variety of chemical substances. Usually, the parametrization of the 2 building block parameters is carried out with a limited number of components and the obtained force field parameters are then transferred to other substances containing the same building blocks. Therefore, the development of transferable force fields is a demanding and tedious task. Transferable force fields can be considered as a construction plan for building component-specific force fields. Thus, transferable force fields are powerful tools that are today widely used in the molecular simulation community. Among transferable force fields, there are different approaches to model the atomistic architecture. Three types of force fields are often distinguished: all-atom (AA), united-atom (UA), and coarse-grained (CG) force fields.35In AA force fields, all atoms are represented by individual interaction sites. In contrast, UA force fields combine multiple atoms to one interaction site. Mostly, CH xgroups are modeled as one interaction site in which the H atoms are only implicitly modeled. In CG force fields, larger molecule segments (or multiple molecules) are merged into a single interaction site. The strength of transferable force fields lies in their generalized description of molecular sub-structuresusingbuildingblocks. However, theflexibilityandwideapplicabilitygenerally comes at the cost of a loss of accuracy in the description compared to component-specific models. Moreover,transferableforcefieldscompriseahighlevelofabstractionandchallenges in the usability.27To the best of our knowledge, no broad data base for transferable force fields of fluids has been established yet. Some force field databases are available,36–39but they mostly cover just one specific transferable force field (usually developed by the creators of the respective database) or potential models for solid states.40 Therefore, in this work, a broad database for transferable force fields was developed and implemented in the existing MolMod database,41which so far only considered component- specific force fields. Fig. 1 gives an overview of the structure of the MolMod Database including the ’component specific’ part as well as the ’transferable’ part presented in this work. The two parts share the same front end, but have individual back end kernels and data structures. 3 TheMolModTransferable databaseisabletohandlebothAAandUAforcefields. Table1 gives an overview of the transferable force fields that are presently implemented in the MolMod database. Presently, MolMod Transferable contains eight transferable force fields, namely: TraPPE,31TAMie,42Potoff,43COMPASS,44CHARMM,45GROMOS,46OPLS- AA,32and OPLS-UA.47These force fields were taken from the literature and cover in general different chemical substance classes (details are given below). The data infrastructure of MolModTransferable usestheforcefielddataontologyanddataformatproposedinRefs.48–50 Based on these interoperable data standards, further transferable force fields can readily be integrated into the MolMod database. A graphical user interface is provided on the website that can be used for defining a molecular structure. The websites directly displays the transferableforcefieldparametersforthatgivenmoleculebasedonachosentransferableforce field. Moreover, MolModTransferable providesinputfilesfordifferentsimulationenginesthat can be downloaded in different formats. This paper is organized as follows: First, the operating principles of MolMod Transferable are explained. Then, the implementation of the force fields and the available input formats are described. Afterwards, comments on the data infrastructure, data management, and data integrity are given. Finally, the work is summarized and an outlook to future work is presented. MolMod Transferable TheMolMod Transferable database can be accessed at https://molmod.boltzmann-zuse. de/. Thepurposeof MolModTransferable istoprovidefindable,accessible,andinteroperable access to force field models for molecules based on transferable force fields. The main task of MolMod Transferable is to build a component-specific force field for a given molecule based on a given transferable force field and provide simulation engine input files. The workflow ofMolMod Transferable is depicted in Fig. 2. The user input (left side of Fig. 2) consists 4 of the specification of the molecule and the transferable force field to be used. The molecule specification is to be provided by specifying the molecule structure, i.e. the geometry of the atoms. The geometry of the molecule can be specified by a drawing of the structural formula orbyprovidingaMOLfile.51ThedrawingcanbedonebytheChemDoodlesketcher,52which is integrated into the MolMod website. Additionally, the user has to select a transferable force field among those currently available in the MolMod database. MolMod displays the component-specific force field model for the considered molecule on-the-fly on the front end of the website. For identifying the functional groups of a given molecule, an algorithm based on the method proposed by Ertl53was implemented inMolMod Transferable . The kernel of MolMod Transferable builds a component-specific force field model by assigning the model parameters from the generalized transferable con- struction plans to a concrete molecular structure. Presently, MolMod Transferable covers only molecules that can be represented by the scheme of Kanagalingam et al. ,50which com- prises a large number organic substances. As all force fields considered here (see below for details) are based on groups that are covered in the scheme of Kanagalingam et al.50If this is the case, the component-specific force field can be built; elsewise the user is informed by the website that the transferable force field cannot be used in the present case. Changes of the molecular structure via the structure drawing tool or changing the transferable force field are quasi-instantaneously depicted on the website. Both, the data of the force field and the meta-data are displayed on the website. The actual force field data consists of the force field parameters, i.e. variables and their numeric values and units that define the interaction sites, the initial configuration, and the intermolecular as well as the intramolecular poten- tials acting between the interaction sites. The meta-data of a user-built component-specific force field comprises the information on the