AndreaBozzo/ceres-open-data-index huggingface dataset:
Unified mechanistic understanding of CO2 reduction to CO on transition metal and single atom catalysts

CO is the simplest product from CO2 electroreduction (CO2R), but the identity and nature of its rate limiting step remains controversial. Here we investigate the activity of both transition metals (TMs) and metal-nitrogen doped carbon catalysts (MNCs), and a present unified mechanistic picture of CO2R for both these classes of catalysts. By consideration of the electronic structure through a Newns-Andersen model, we find that on MNCs, like TMs, electron transfer to CO2 is facile, such that CO2 (g) adsorption is driven by adsorbate dipole-field interactions. Using density functional theory with explicit consideration of the interfacial field, we find CO2 * adsorption to generally be limiting on TMs, while MNCs can be limited by either CO2* adsorption or by the proton-electron transfer reaction to form COOH*. We evaluate these computed mechanisms against pH-dependent experimental activity measurements on CO2R to CO activity for Au, FeNC, and NiNC. We present a unified activity volcano that, in contrast to previous analyses, includes the decisive CO2* and COOH* binding strengths as well as the critical adsorbate dipole-field interactions. This dataset contains all DFT calculations used in this work.

Tags: catalysis, dipole-field, electrocataylsis, h2020
Impact of quantum-chemical metrics on the machine learning prediction of electron density

Machine learning (ML) algorithms have undergone an explosive development impacting every aspect of computational chemistry. To obtain reliable predictions, one needs to maintain the proper balance between the black-box nature of ML frameworks and the physics of the target properties. One of the most appealing quantum-chemical properties for regression models is the electron density, and some of us recently proposed a transferable and scalable model based on the decomposition of the density onto an atom-centered basis set. The decomposition, as well as the training of the model, is at its core a minimization of some loss function, which can be arbitrarily chosen and may lead to results of different quality. Well-studied in the context of density fitting (DF), the impact of the metric on the performance of ML models has not been analyzed yet. In this work, we compare predictions obtained using the overlap and the Coulomb repulsion metrics for both the decomposition and training. As expected, the Coulomb metric used as both the DF and ML loss functions leads to the best results for the electrostatic potential and dipole moments. The origin of this difference lies in the fact that the model is not constrained to predict densities that integrate to the exact number of electrons N. Since an a posteriori correction for the number of electrons decreases the errors, we proposed a modification of the model where N is included directly into the kernel function, which allowed to lower the errors on the test and out-of-sample sets.

Tags: correction-number-of-electrons, epfl, erc, machine-learning, marveldd1, quantum-chemical-metrics, snsf
Magnetic and electronic properties at the γ-Al2O3/SrTiO3 interface

The magnetic and electronic nature of the γ-Al2O3/SrTiO3 spinel/perovskite interface is explored by means of x-ray absorption spectroscopy. Polarized x-ray techniques combined with atomic multiplet calculations reveal localized magnetic moments assigned to Ti3+ at the interface with equivalent size for in- and out-of-plane magnetic field directions. Although magnetic fingerprints are revealed, the Ti3+ magnetism can be explained by a paramagnetic response at low temperature under applied magnetic fields. Modeling the x-ray linear dichroism results in a Delta0 ∼ 1.9 eV splitting between the t2g and eg states for the Ti4+ 3d0 orbitals. In addition these results indicate that the lowest energy states have the out-of-plane dxz/dyz symmetry. The isotropic magnetic moment behavior and the lowest energy dxz/dyz states are in contrast to the observations for the two-dimensional electron gas at the perovskite/perovskite interface of LaAlO3/SrTiO3 that exhibits an anisotropic magnetic dxy ground state.

Tags: experimental, magnetism, two-dimensional-electron-gas, x-ray-absorption-spectroscopy
Observation of fractional edge excitations in nanographene spin chains

Fractionalization is a phenomenon in which strong interactions in a quantum system drive the emergence of excitations with quantum numbers that are absent in the building blocks. Outstanding examples are excitations with charge e/3 in the fractional quantum Hall effect, solitons in one-dimensional conducting polymers and Majorana states in topological superconductors. Fractionalization is also predicted to manifest itself in low-dimensional quantum magnets, such as one-dimensional antiferromagnetic S = 1 chains. The fundamental features of this system are gapped excitations in the bulk and, remarkably, S = 1/2 edge states at the chain termini, leading to a four-fold degenerate ground state that reflects the underlying symmetry-protected topological order. This record contains data to support the result in a recent publication of ours where we use on-surface synthesis to fabricate one-dimensional spin chains that contain the S = 1 polycyclic aromatic hydrocarbon triangulene as the building block. Using scanning tunneling microscopy and spectroscopy at 4.5 K, we probe length-dependent magnetic excitations at the atomic scale in both open-ended and cyclic spin chains, and directly observe gapped spin excitations and fractional edge states therein. Exact diagonalization calculations provide conclusive evidence that the spin chains are described by the S = 1 bilinear-biquadratic Hamiltonian in the Haldane symmetry-protected topological phase. Our results open a bottom-up approach to study strongly correlated phases in purely organic materials, with the potential for the realization of measurement-based quantum computation.

Tags: cscs, erc, fractionalization, graphene-flagship, h2020, marveldd3, nanographene, snsf, spin-chains
Accurate electronic properties and intercalation voltages of olivine-type Li-ion cathode materials from extended Hubbard functionals

The design of novel cathode materials for Li-ion batteries requires accurate first-principles predictions of structural, electronic, and magnetic properties as well as intercalation voltages in compounds containing transition-metal (TM) elements. For such systems, density-functional theory (DFT) with standard (semi-)local exchange-correlation functionals is of limited use as it often fails due to strong self-interaction (delocalization) errors that are especially large for the partially filled d shells of the TMs. Here, we perform the first comparative study of the phospho-olivine cathode materials LixMnPO4, LixFePO4, and mixed-TM LixMn1/2Fe1/2PO4 (x=0, 1/4, 1/2, 3/4, 1) using four electronic structure methods: DFT, DFT+U, DFT+U+V, and HSE06. We show that DFT+U+V outperforms the other three methods, provided that the onsite U and intersite V Hubbard parameters are determined from first-principles and self-consistently with respect to the structural parameters by means of density-functional perturbation theory (linear response). In particular, we demonstrate that DFT+U+V is the only method that correctly predicts the digital change in oxidation states of the TM ions in all compounds for the mixed-valence phases occurring at intermediate Li concentrations, leading to voltages in remarkable agreement with experiments. We thus show that the inclusion of intersite Hubbard interactions is essential for the accurate prediction of thermodynamic quantities when electronic localization occurs while in the presence of inter-atomic orbital hybridization. At variance with the other methods, DFT+U+V alone is capable to describe such localization-hybridization interplay, and thus opens the door for the study of more complex cathode materials as well as for a reliable exploration of the chemical space of compounds for Li-ion batteries.

Tags: cscs, dftu, dftuv, extended-hubbard-functionals, first-principles-calculations, formation-energy, hse06, hybrid-functionals, intercalation-voltages, li-ion-batteries, marvelosp, phospho-olivine-cathode-materials
Controlled selectivity for ethanol steam reforming reaction over doped CeO2 surfaces: The role of gallium

The ethanol steam reforming reaction, together with the adsorption and decomposition of ethanol was studied on CeO2 and gallium-doped ceria (CeGaOx) by a combined experimental and theoretical approach using infrared spectroscopy (IR), mass spectrometry (MS) and density functional theory (DFT) calculations. At 100 °C, different types of monodentate ethoxy species were identified as standing-up (SU) on Ce4+ and lying-down (LD) on Ce4+ and Ga3+, with the alkyl chain more perpendicular or parallel to the surface, respectively. It is suggested that the incorporation of Ga into the ceria lattice changes the decomposition pathway of LD species, which converts to acetate instead of ethylene, attributed to the increased lattice oxygen lability in the Ce―O―Ga interface upon doping and the propensity to form Ga―H surface species. Under ethanol steam reforming conditions, Ga doping of ceria-based materials has a drastic effect by improving the H2:CO2 ratio, changing the product distribution and reducing coke formation.

Tags: bioethanol, coke-precursors, deactivation, gallia, hydrogen-production, prace
Light-induced renormalization of the Dirac quasiparticles in the nodal-line semimetal ZrSiSe

In nodal-line semimetals, linearly dispersing states form Dirac loops in the reciprocal space with a high degree of electron-hole symmetry and a reduced density of states near the Fermi level. The result is reduced electronic screening and enhanced correlations between Dirac quasiparticles. Here we investigate the electronic structure of ZrSiSe, by combining time- and angle-resolved photoelectron spectroscopy with ab initio density functional theory (DFT) complemented by an extended Hubbard model (DFT+U+V) and by time-dependent DFT+U+V. We show that electronic correlations are reduced on an ultrashort timescale by optical excitation of high-energy electrons-hole pairs, which transiently screen the Coulomb interaction. Our findings demonstrate an all-optical method for engineering the band structure of a quantum material.

This record contains the ARPES raw data in txt format used to create the figures in the referenced publication.

Tags: electronic-structure, erc, experimental, snsf, topological-materials, trarpes
Electron energy loss spectroscopy of bulk gold with ultrasoft pseudopotentials and the Liouville-Lanczos method

The implementation of ultrasoft pseudopotentials into time-dependent density-functional perturbation theory is detailed for both the Sternheimer approach and the Liouville-Lanczos (LL) method, and equations are presented in the scalar relativistic approximation for periodic solids with finite momentum transfer q. The LL method is applied to calculations of the electron energy loss (EEL) spectrum of face-centered cubic bulk Au both at vanishing and finite q. Our study reveals the richness of the physics underlying the various contributions to the density fluctuation in gold. In particular, our calculations suggest the existence in gold of two quasiseparate 5d and 6s electron gasses, each one oscillating with its own frequency at, respectively, 5.1 eV and 10.2 eV. We find that the contribution near 2.2 eV comes from 5d to 6s interband transitions modified by the intraband contribution to the real part of the dielectric function, which we call a mixed excitation.

Tags: dft, eels, eu-max, genci, gold, interband-transitions, intraband-transitions, liouville-lanczos-method, loss-function, marvelosp, mixed-excitations, plasmons, quantum-espresso, sternheimer-equations, tddft
ONETEP + TOSCAM: uniting dynamical mean field theory and linear-scaling density functional theory

We introduce the unification of dynamical mean field theory (DMFT) and linear-scaling density functional theory (DFT), as recently implemented in ONETEP, a linear-scaling DFT package, and TOSCAM, a DMFT toolbox. This code can account for strongly correlated electronic behavior while simultaneously including the effects of the environment, making it ideally suited for studying complex and heterogeneous systems that contain transition metals and lanthanides, such as metalloproteins. We systematically introduce the necessary formalism, which must account for the non-orthogonal basis set used by ONETEP. In order to demonstrate the capabilities of this code, we apply it to carbon monoxide-ligated iron porphyrin and explore the distinctly quantum-mechanical character of the iron 3d electrons during the process of photodissociation.

This archive record contains example input and output files for the DFT, DFT+U, and DFT+DMFT calculations presented in the associated journal article.

Tags: density-functional-theory, dynamical-mean-field-theory, linear-scaling-dft
Temperature dependence of electronic eigenenergies in the adiabatic harmonic approximation

The renormalization of electronic eigenenergies due to electron-phonon interactions (temperature dependence and zero-point motion effect) is important in many materials. We address it in the adiabatic harmonic approximation, based on first principles (e.g., density-functional theory), from different points of view: directly from atomic position fluctuations or, alternatively, from Janak's theorem generalized to the case where the Helmholtz free energy, including the vibrational entropy, is used. We prove their equivalence, based on the usual form of Janak's theorem and on the dynamical equation. We then also place the Allen-Heine-Cardona (AHC) theory of the renormalization in a first-principles context. The AHC theory relies on the rigid-ion approximation, and naturally leads to a self-energy (Fan) contribution and a Debye-Waller contribution. Such a splitting can also be done for the complete harmonic adiabatic expression, in which the rigid-ion approximation is not required. A numerical study within the density-functional perturbation theory framework allows us to compare the AHC theory with frozen-phonon calculations, with or without the rigid-ion approximation. For the two different numerical approaches without non-rigid-ion terms, the agreement is better than 7 μeV in the case of diamond, which represent an agreement to five significant digits. The magnitude of the non-rigid-ion terms in this case is also presented, distinguishing specific phonon modes contributions to different electronic eigenenergies.

Tags: ab-initio, adiabatic-harmonic-approximation, allen-heine-cardona-theory, ceci, diamond, electron-phonon-coupling, first-principles, frqnt, frs-fnrs, temperature-dependence, verification-and-validation, zero-point-motion-renormalization
Automated all-functionals infrared and Raman spectra

Infrared and Raman spectroscopies are ubiquitous techniques employed in many experimental laboratories, thanks to their fast and non-destructive nature able to capture materials' features as spectroscopic fingerprints. Nevertheless, these measurements frequently need theoretical support in order to unambiguously decipher and assign complex spectra. Linear-response theory provides an effective way to obtain the higher-order derivatives needed, but its applicability to modern exchange-correlation functionals remains limited. Here, we devise an automated, open-source, user-friendly approach based on ground-state density-functional theory and the electric enthalpy functional to allow seamless calculations of first-principles infrared and Raman spectra. By employing a finite-displacement and finite-field approach, we allow for the use of any functional, as well as an efficient treatment of large low-symmetry structures. Additionally, we propose a simple scheme for efficiently sampling the Brillouin zone with different electric fields. To demonstrate the capabilities of our approach, we provide illustrations using the ferroelectric LiNbO₃ crystal as a paradigmatic example. We predict infrared and Raman spectra using various (semi)local, Hubbard corrected, and hybrid functionals. Our results also show how PBE0 and extended Hubbard functionals yield in this case the best match in term of peak positions and intensities, respectively.

Tags: ab-initio, aiida, automated, density-functional-theory, infrared, phonons, raman, vibrational-properties
Revised MD17 dataset

The original MD17 dataset (http://quantum-machine.org/datasets/#md-datasets) [Chemiela et al. Sci. Adv. 3(5), e1603015, 2017] contains numerical noise. Thus, any numbers presented from benchmarks on this data are likely flawed. Here, we present a new dataset with negligible numerical noise for benchmarking of forces and energy predictions for molecular dynamics simulations. As the structures are taken from a molecular dynamics simulation (i.e. time series data), they are not guaranteed to be independent samples. This is easily evident from the autocorrelation function for the original MD17 dataset. In short: DO NOT train a model on more than 1000 samples from the revised dataset, and do not train models for more than 50 samples from the original MD17 dataset. Data already published with 50K samples on the original MD17 dataset should be considered meaningless due to this fact and due to the noise in the original data.

Tags: chemistry, energies, forces, machine-learning, molecules, noise
Screening from eg states and antiferromagnetic correlations in d(1,2,3) perovskites: A GW+EDMFT investigation

We perform a systematic ab initio study of the electronic structure of Sr(V,Mo,Mn)O3 perovskites, using the parameter-free GW+EDMFT method. This approach self-consistently calculates effective interaction parameters, taking into account screening effects due to nonlocal charge fluctuations. Comparing the results of a 3-band (t2g) description to those of a  5-band (t2g+eg) model, it is shown that the eg states have little effect on the low-energy properties and the plasmonic features for the first two compounds but play a more active role in SrMnO3. In the case of SrMnO3 paramagnetic GW+EDMFT yields a metallic low-temperature solution on the verge of a Mott transition, while antiferromagnetic GW+EDMFT produces an insulating solution with the correct gap size. We discuss the possible implications of this result for the nature of the insulating state above the Neel temperature, and the reliability of the GW+EDMFT scheme.

Tags: correlated-oxides, dynamical-mean-field-theory, erc, gwedmft, marveldd5
Hidden order and multipolar exchange striction in a correlated f-electron system

The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such "hidden orders" (HO) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f or d shells that may harbor high-rank multipole moments. Coupled by inter-site exchange, those moments form a vast space of competing order parameters. Here, we show how the ground state order and magnetic excitations of a prototypical HO system, neptunium dioxide NpO2, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force theorem method. Superexchange interactions between the lowest crystal-field quadruplet of Np4+ ions induce a primary non-collinear order of time-odd rank-5 (triakontadipolar) moments with a secondary quadrupole order preserving the cubic symmetry of NpO2. Our study also reveals an unconventional multipolar exchange-striction mechanism behind the anomalous volume contraction of the NpO2 HO phase.

Tags: ab-initio, effective-hamiltonians, erc, hidden-order, linear-response, strongly-correlated-electrons
Pyrene-based metal organic frameworks

Pyrene is one of the most widely investigated aromatic hydrocarbons due to its unique optical and electronic properties. Hence, pyrene-based ligands have been investigated for the synthesis of metal-organic frameworks (MOFs) in the last few years. This dataset collects the atomic structures of the pyrene-based MOFs discussed in Table 1 of Ref. 1. 

The crystal structures have been manually curated to resolve partial occupancies and remove solvent molecules. Charge-neutral structures were optimized using DFT following the CURATED protocol described in 10.1021/acscentsci.9b00619.

Tags: erc, marveldd4, metal-organic-framework, mof, pyrene, snsf
One-shot approach for enforcing piecewise linearity on hybrid functionals: application to band-gap predictions

We present an efficient procedure for constructing nonempirical hybrid functionals to accurately predict band gaps of extended systems. We determine mixing parameters by enforcing the generalized Koopmans' condition on localized electron states, which are achieved by inserting an optimized potential probe. Application of this scheme to a large set of materials yields band gaps with a mean error of 0.30 eV with respect to experiment. Next, we consider a perturbative one-shot approach in which the single- particle eigenvalues are calculated with the wave functions obtained at the semilocal level. In this way, the computational cost is reduced by ∼85% without loss of accuracy. The scheme is found to be robust upon consideration of different defect species and functional forms.

Tags: density-functional-theory, marveldd4, pseudopotentials, quantum-espresso, snsf
Identifying the trade-off between intramolecular singlet fission requirements in donor-acceptor copolymers

Intramolecular singlet fission (iSF) has shown potential to improve the power conversion efficiency in photovoltaic devices by promoting the splitting of a photon-absorbing singlet exciton into two triplet excitons within a single molecule. Among different possibilities, the donor-acceptor modular strategy of copolymers has shown great promise in its ability to undergo iSF under certain conditions. However, the number of iSF donor-acceptor copolymers reported in the literature remains remarkably narrow and clear trends for the molecular design of better candidates have not yet been established. In this work, we identify the trade-off between the main iSF requirements of the donor-acceptor strategy and formulate design rules that allow them to be tuned simultaneously in a fragment-based approach. Based on a library of 2944 donor-acceptor copolymers, we establish simple guidelines to build promising novel materials for iSF. These consist in (1st) selecting an acceptor core with high intrinsic singlet-triplet splitting, (2nd) locating a donor with a larger monomer frontier molecular orbital (FMO) gap than that of the acceptor, and (3rd) tuning the relative energy of donor and/or acceptor FMOs through functionalization to promote photoinduced charge transfer in the resulting polymer. Remarkably, systems containing benzothiadiazole and thiophene-1,1-dioxide acceptors, which have been shown to undergo iSF, fulfill all criteria simultaneously when paired with appropriate donors. This is due to their particular electronic features, which make them highly promising candidates in the quest for iSF materials.

Tags: conjugated-polymers, epfl, excited-states, singlet-fission