BIGDML: Towards Exact Machine Learning Force Fields for Materials

Machine-learning force fields (MLFF) should be accurate, computationally and data efficient, and applicable to molecules, materials, and interfaces thereof. Currently, MLFFs often introduce tradeoffs that restrict their practical applicability to small subsets of chemical space or require exhaustive datasets for training. Here, we introduce the Bravais-Inspired Gradient-Domain Machine Learning (BIGDML) approach and demonstrate its ability to construct reliable force fields using a training set with just 10-200 geometries for materials including pristine and defect-containing 2D and 3D semiconductors and metals, as well as chemisorbed and physisorbed atomic and molecular adsorbates on surfaces. The BIGDML model employs the full relevant symmetry group for a given material, does not assume artificial atom types or localization of atomic interactions and exhibits high data efficiency and state-of-the-art energy accuracies (errors substantially below 1 meV per atom) for an extended set of materials. Extensive path-integral molecular dynamics carried out with BIGDML models demonstrate the counterintuitive localization of benzene--graphene dynamics induced by nuclear quantum effects and allow to rationalize the Arrhenius behavior of hydrogen diffusion coefficient in a Pd crystal for a wide range of temperatures.Comment: 15 pages, 8 figures, development of methodology and application

Effects of atomic scale roughness at metal/insulator interfaces on metal work function

We evaluate the performance of different van der Waals (vdW) corrected density functional theory (DFT) methods in predicting the structure of perfect interfaces between the LiF(001), MgO(001), NiO(001) films on the Ag(001) surface and the resulting work function shift of Ag(001). The results demonstrate that including the van der Waals interaction is important for obtaining accurate interface structures and the metal work function shift. The work function shift results from a subtle interplay of several effects strongly affected by even small changes in the interface geometry. This makes the accuracy of theoretical methods insufficient for predicting the shift values better than within 0.2 eV. Most of the existing van der Waals corrected functionals are not particularly suited for studying metal/insulator interfaces. The lack of accurate experimental data on the interface geometries and surface rumpling of insulators hampers the calibration of existing and novel density functionals

A DFT Study on the O2 Adsorption Properties of Supported PtNi Clusters

We present a systematic study on the adsorption properties of molecular oxygen on Pt, Ni and PtNi clusters previously deposited on MgO(100) by means of density functional theory calculations. We map the different adsorption sites for a variety of cluster geometries, including icosahedra, decahedra, truncated octahedra and cuboctahedra, in the size range between 25–58 atoms. The average adsorption energy depends on the chemical composition, varying from 2 eV for pure Ni, 1.07 for pure Pt and 1.09 for a Pt s h e l l Ni c o r e nanoalloy. To correlate the adsorption map to the adsorption properties, we opt for a geometrical descriptor based on the metallic coordination up to the second coordination shell. We find an almost linear relationship between the second coordination shell and adsorption energy, with low coordination sites, such as those located at the (111)/(111) and (111)/(100) cluster edges-displaying adsorption energies above 1 eV, while higher coordination sites such as (111) cluster facets have an interaction of 0.4 eV or lower. The inclusion of van der Waals corrections leads to an overall increase of the O 2 adsorption energy without an alteration of the general adsorption trends

Revista nacional de educación

Exposición de la admiración que el ilustre dramaturgo Calderón de la Barca provoca en Alemania, siendo muchas las traducciones que se han hecho de sus obras al alemán, y muchos los estudios que se han hecho sobre su repertorio en este país.Ministerio Educación CIDEBiblioteca de Educación del Ministerio de Educación, Cultura y Deporte; Calle San Agustín, 5 - 3 Planta; 28014 Madrid; Tel. +34917748000; [email protected]

Melting of large Pt@MgO(1 0 0) icosahedra

On the basis of ab initio calculations, we present a new parametrisation of the Vervisch-Mottet-Goniakowski (VMG) potential (Vervisch et al 2002 Phys. Rev. B 24 245411) for modelling the oxide-metal interaction. Applying this model to mimic the finite temperature behaviour of large platinum icosahedra deposited on the pristine MgO(1 0 0), we find the nanoparticle undergoes two solid-solid transitions. At 650 K the 'squarisation' of the interface layer, while a full reshaping towards a fcc architecture takes place above 950 K. In between, a quite long-lived intermediate state with a (1 0 0) interface but with an icosahedral cap is observed. Our approach reproduces experimental observations, including wetting behaviour and the lack of surface diffusion

Structures and Stabilities of Platinum-Gold Nanoclusters

A genetic algorithm, coupled with the empirical Gupta many-body potential, is used to perform global optimisations for Pt–Au bimetallic clusters, with 1:1 composition, over a broad size range (N = 2−100). By analyzing different stability criteria, such as binding energy (Eb) and second difference in energy (Δ2E) the lowest energy structures, i.e., global minima (GM), were determined. Icosahedron-based structures predominate as GM in the small size regime (up to N = 28). For medium sized clusters, e.g., N = 38, a strong preference for FCC-type structures is found. Moreover, a previously predicted GM structure for N = 98, i.e., the Leary Tetrahedron (LT), was found as the GM for 98-atom Pt–Au clusters. Density Functional Theory (DFT) local geometry optimisations were also performed on smaller clusters (N = 2−20). Changes in cluster geometry were small, though relaxations in bond lengths were observed. At the Gupta potential level, it was found that PtcoreAushell segregation is energetically preferred, which was subsequently confirmed by our DFT calculations for selected nuclearities

AuN clusters (N = 1–6) supported on MgO(100) surfaces: Effect of exact exchange and dispersion interactions on adhesion energies

The energetics of an Au adatom and AuN clusters (N = 2–6) supported on pristine and reduced MgO(100) surfaces is analyzed using an all-electron full-potential density functional theory approach. A hierarchy of exchange-correlation functional approximations is employed, ranging from the generalized gradient approximation [Perdew-Burke-Ernzerhof (PBE), revised PBE (RPBE)] to hybrid functionals [PBE0, Heyd-Scuseria-Ernzerhof (HSE06)] to exact exchange plus correlation in the random phase approximation (EX-cRPA/cRPA+). The analysis of different terms in the electronic Hamiltonian, contributing to calculated adhesion energies (Eadh) for the Au adatom, shows that reducing the self-interaction error leads to smaller Eadh values. On the contrary, the energy barriers for diffusion of an Au adatom at the pristine surface significantly increase. For AuN clusters (N >1), dispersion effects, not accounted for by the generalized gradient approximation or hybrid functionals, start to make an increasingly important contribution to the adhesion energy