Publications

Structural and electronic properties of molten semimetals: An ab initio study for liquid antimony

The structural and electronic properties of the crystalline semimetals As and Sb may be understood in terms of a Peierls distortion from the simple cubic structure bonds: as the (ppσ)-band is exactly half-filled, the cubic structure is unstable against dimerization dividing the six nearest-neighbour bonds into three strong and three weak bonds and opening a deep gap in the electronic density of states at the Fermi level. On the basis of neutron-diffraction experiments it has been suggested that some of the anomalous structural and electronic properties survive in the liquid phase. Here, an ab initio investigation of the structural, electronic, and dynamic properties of molten Sb that is based on the full set of quantum-many-body forces is presented. The results confirm the picture of a Peierls-distortion in the liquid.

Effects of Lattice Expansion on the Reactivity of a One-Dimensional Oxide

By means of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we characterize at the single-atom level the mechanism of the water formation reaction on the (10 x 2)-O/Rh(110) surface, a prototype of a one-dimensional (1D) oxide where the lattice expansion and the segmentation of the surface play a fundamental role. When the reaction is imaged in the 238-263 K temperature range (35 s/image acquisition time), a peculiar comblike propagation mechanism for the reaction front is found. Fast STM measurements (33 ms/image) prove that this mechanism holds also at room temperature, being therefore an intrinsic characteristic of the reaction on the 1D oxide. DFT calculations explain the observed behavior as due to the interplay between the lattice expansion in the initial surface and its relaxation during the reaction that leads to varying configurations for the reactants. At low temperatures, the reaction produces, in its final stages, a low-coverage, ordered patterning of the surface with residual oxygen. The pattern formation is related to the segmentation of the oxide phase.

On-the-fly machine learning force field generation: Application to melting points

An efficient and robust on-the-fly machine learning force field method is developed and integrated into an electronic-structure code. This method realizes automatic generation of machine learning force fields on the basis of Bayesian inference during molecular dynamics simulations, where the first principles calculations are only executed, when new configurations out of already sampled datasets appear. The developed method is applied to the calculation of melting points of Al, Si, Ge, Sn and MgO. The applications indicate that more than 99 \% of the first principles calculations are bypassed during the force field generation. This allows the machine to quickly construct first principles datasets over wide phase spaces. Furthermore, with the help of the generated machine learning force fields, simulations are accelerated by a factor of thousand compared with first principles calculations. Accuracies of the melting points calculated by the force fields are examined by thermodynamic perturbation theory, and the examination indicates that the machine learning force fields can quantitatively reproduce the first principles melting points.

Merging GW with DMFT and non-local correlations beyond

We review recent developments in electronic structure calculations that go beyond state-of-the-art methods such as density functional theory (DFT) and dynamical mean field theory (DMFT). Specifically, we discuss the following methods: GW as implemented in the Vienna ab initio simulation package (VASP) with the self energy on the imaginary frequency axis, GW+DMFT, and ab initio dynamical vertex approximation (DΓA). The latter includes the physics of GW, DMFT and non-local correlations beyond, and allows for calculating (quantum) critical exponents. We present results obtained by the three methods with a focus on the benchmark material SrVO3.

<i>Ab initio</i>molecular-dynamics simulation of the liquid-metal–amorphous-semiconductor transition in germanium

We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.

Self-limited growth of a thin oxide layer on Rh(111): A DFT study

Oxidation is often associated with corrosion, but under the right conditions it can lead to oxide layers which can be applied e.g. as protective layers against corrosion, as insulating layers in microelectronic devices and as catalytic devices[1, 2]. For late transition metals and noble metals (e.g. Pd and Ag), it is now understood that the oxidation proceeds through ultra-thin oxide layers, which are thermodynamically stable at intermediate oxygen potentials and can exhibit astonishing complexity[3–6]. It is still unclear whether the same holds for transition metals and to this end the Rh (111) surface was studied extensively experimentally [7]. To supplement these experimental studies, we performed density functional calculations with the Vienna Ab Initio Simulation Package (VASP) [8] using plane waves and the PAW method [9] as well as generalized gradient approximations [10]. The experimental studies show a Moire like pattern in STM, indicative of a (8×8) oxide layer on a (9×9) supercell of the Rh(111) substrate. This phase is formed at intermediate oxygen pressures, whereas thick corundum-like Rh2O3 is only formed at significantly higher pressures and temperatures. The theoretical calculations indicate that the Moire phase can be rationalised by an ultrathin O-Rh-O trilayer surface oxide. Contrary to the experimental observation, the bulk Rh2O3 oxide is however thermodynamically more stable than ultra-thin layers. This discrepency can be understood by the surface phase diagram shown in Fig. 1(a). It indicates that a single oxygen trilayer on Rh(111) is in fact only metastable, as the trilayer forms only under conditions, where bulk Rh2O3 is already stable (to the right of the thick grey line in Fig. 1(a)). Hence the trilayer is only kinetically stabilised, contrary to the situation on Pd(111) where a Pd5O4 ad-layer is thermodynamically stable for intermediate oxygen potentials [4]. An important hint to the reason of the kinetic stability of the O-Rh-O trilayer is given by the results for the three and four layer thick oxides. Four (three) layer oxides have a lower stability than the single trilayer for oxygen potentials μO <−0.99 eV (μO <−0.78 eV) corresponding to 10 mbar (1 bar) at 800 K. The formation of the bulk oxide must however proceed through thicker oxide layers that present a kinetic barrier for the formation of the bulk oxide at too low chemical potentials. The most favorable structures for 2, 3 and 4 oxygen layers are depicted in Fig. 1(b). In contrast to oxygen atoms on the clean metal, the oxygen atoms at the oxide/metal interface are located preferentially on top of the surface Rh atoms (shifted slightly towards the bridge site). Remarkably, the trilayer termination remains favorable even for thicker oxides. For three oxygen layers (3L) the topmost surface layer contains three Rh atoms, and a fourth single Rh atom is located in the second oxide layer. For four oxygen layers (4L), the trilayer is found at both sides of the oxide, and a single Rh atom interlinks the two O-Rh-O layers. Poster

Phaseless auxiliary field quantum Monte Carlo with projector-augmented wave method for solids

We implement the phaseless auxiliary field quantum Monte Carlo method using the plane-wave based projector augmented wave method and explore the accuracy and the feasibility of applying our implementation to solids. We use a singular value decomposition to compress the two-body Hamiltonian and, thus, reduce the computational cost. Consistent correlation energies from the primitive-cell sampling and the corresponding supercell calculations numerically verify our implementation. We calculate the equation of state for diamond and the correlation energies for a range of prototypical solid materials. A down-sampling technique along with natural orbitals accelerates the convergence with respect to the number of orbitals and crystal momentum points. We illustrate the competitiveness of our implementation in accuracy and computational cost for dense crystal momentum point meshes compared to a well-established quantum-chemistry approach, the coupled-cluster ansatz including singles, doubles, and perturbative triple particle-hole excitation operators.

Ground-state properties of multivalent manganese oxides: Density functional and hybrid density functional calculations

We present density functional theory (DFT) calculations for MnO, ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$, and $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Mn}{\mathrm{O}}_{2}$, using different gradient corrected functionals, such as Perdew-Burke-Ernzerhof (PBE), $\mathrm{PBE}+\mathrm{U}$, and the two hybrid density functional Hartree-Fock methods PBE0 and Heyd-Scuseria-Ernzerhof (HSE). We investigate the structural, electronic, magnetic, and thermodynamical properties of the mentioned compounds. Despite the lack of sufficient experimental information allowing for a comprehensive comparison of our results, we find overall that hybrid functionals provide a more consistent picture than standard PBE. Although $\mathrm{PBE}+\mathrm{U}$ is limited due to the uncertainty of choosing the parameter U, it nevertheless provides satisfactory results in terms of magnetic properties and energies of formation. This is in line with results of PBE0 and HSE calculations, but the $\mathrm{PBE}+\mathrm{U}$ approach tends to overestimate the equilibrium volumes, and also it favors a half-metallic state for the more reduced oxides ${\mathrm{Mn}}_{3}{\mathrm{O}}_{4}$, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Mn}}_{2}{\mathrm{O}}_{3}$, and $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{Mn}{\mathrm{O}}_{2}$, rather than an insulating character as derived from the hybrid functional approaches. The comparison of measured valence-band spectra with the HSE density of states offers a further assessment of the capability of hybrid approaches in overcoming the deficiencies of DFT in treating these kinds of materials.

Singles correlation energy contributions in solids

The random phase approximation to the correlation energy often yields highly accurate results for condensed matter systems. However, ways how to improve its accuracy are being sought and here we explore the relevance of singles contributions for prototypical solid state systems. We set out with a derivation of the random phase approximation using the adiabatic connection and fluctuation dissipation theorem, but contrary to the most commonly used derivation, the density is allowed to vary along the coupling constant integral. This yields results closely paralleling standard perturbation theory. We re-derive the standard singles of Görling-Levy perturbation theory [A. Görling and M. Levy, Phys. Rev. A 50, 196 (1994)], highlight the analogy of our expression to the renormalized singles introduced by Ren and coworkers [Phys. Rev. Lett. 106, 153003 (2011)], and introduce a new approximation for the singles using the density matrix in the random phase approximation. We discuss the physical relevance and importance of singles alongside illustrative examples of simple weakly bonded systems, including rare gas solids (Ne, Ar, Xe), ice, adsorption of water on NaCl, and solid benzene. The effect of singles on covalently and metallically bonded systems is also discussed.

First Principles Modeling of the Growth of Crystalline Oxides on Silicon: The First Two Monolayers

No abstract is provided for this article.

PhysRevB

Ab-initio Study on the Adsorption of H, H 2 and CO on the Reduced Ceria CeO_2-x (111) Surface

New insights into the 1D carbon chain through the RPA

Using correlated wave function based methods, the modeling of promising new materials is elevated to a new level. For the first time, a realistic phonon dispersion relation is predicted for the infinite linear carbon chain.

Structure, Energetics, and Electronic Properties of the Surface of a Promoted MoS2 Catalyst: An ab Initio Local Density Functional Study

The determination of the local structure of cobalt- or nickel-promoted MoS2-based hydrodesulfurization catalysts is of interest for understanding the mechanism leading to an increased activity brought by cobalt or nickel, the so-called synergetic effect. For that reason, we carried out ab initio calculations using density functional theory under the generalized gradient approximation for periodic systems. The edge substitution model emerges as the most stable structure and provides an excellent agreement with local structures experimentally determined on real catalysts by in situ extended X-ray absorption fine structure. We studied the adsorption of sulfur on the active edge surface of the promoted MoS2 catalyst and determined the equilibrium coverage under sulfiding conditions. It is demonstrated that the incorporation of promoter atoms has a strong influence on the sulfur–metal bond energy at the surface and in particular leads to a reduction of the equilibrium S coverage of the active metal sites. A comparative study on the effects of Co, Ni, and Cu atoms as promoters was performed. Detailed results on the surface electronic structure of promoted MoS2 are presented.

<i>Ab initio</i>density functional studies of transition-metal sulphides: II. Electronic structure

A study of the electronic structure of about thirty transition-metal sulphides (TMS) of various stoichiometries and crystal structures is presented, supplementing recent studies of their structural and cohesive properties (P Raybaud, G Kresse, J Hafner and H Toulhoat, preceding paper). The electronic structure of the TMS is found to be determined by short-range interactions in the S 3p - TM d band complex, with the ligand-field splitting of the TM d states in the environment of the S atoms determining the structure of the d band. For the layered group VI disulphides, for and for the group VIII pyrites this leads to the formation of a gap at the Fermi surface. Semiconducting properties are predicted also for the monosulphides PtS and PdS and for and . We show that the semiconducting TMS have a higher catalytic activity for hydro-desulphurization than the metallic sulphides. We suggest a correlation between the catalytic activity and the characters of the highest occupied states (the frontier orbitals).

Hidden correlation and quasiparticle spectra in valence-skipper Peierls semiconductor $\rm BaBiO_3$ from self-consistent GW

$\rm BaBiO_3$ is characterized by a charge disproportionation with half of the Bi atoms possessing a valence 3+ and half a valence 5+. Because of selfinteraction errors, local and semi-local density functionals fail to describe the charge disproportionation quantitatively, yielding a too small structural distortion and no band gap. Using hybrid functionals we obtain a satisfactory description of the structural, electronic, optical, and vibrational properties of $\rm BaBiO_3$. The results obtained using GW (Green's function G and screened Coulomb potential W) based schemes on top of hybrid functionals, including fully selfconsistent GW calculations with vertex corrections in the dielectric screening, qualitatively confirm the HSE picture but a systematic overestimation of the bandgap by about 0.4 eV is observed.

Non-local first-principles calculations in Cu-Au, Ag-Au and Cu-Ag

No abstract is provided for this article.

Assessing model-dielectric-dependent hybrid functionals on the antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO

Recently, two nonempirical hybrid functionals, dielectric-dependent range-separated hybrid functional based on the Coulomb-attenuating method (DD-RSH-CAM) and doubly screened hybrid functional (DSH), have been suggested by [Chen et al, Phys. Rev. Mater. 2, 073803 (2018)] and [Cui et al, J. Phys. Chem. Lett. 9, 2338 (2018)], respectively. These two hybrid functionals are both based on a common model dielectric function approach, but differ in the way how to non-empirically obtain the range-separation parameter. By retaining the full short-range Fock exchange and a fraction of the long-range Fock exchange that equals the inverse of the dielectric constant, both DD-RSH-CAM and DSH turn out to perform very well in predicting the band gaps for a large variety of semiconductors and insulators. Here, we assess how these two hybrid functionals perform on challenging antiferromagnetic transition-metal monoxides MnO, FeO, CoO, and NiO by comparing them to other conventional hybrid functionals and the $GW$ method. We find that single-shot DD0-RSH-CAM and DSH0 improve the band gaps towards experiments as compared to conventional hybrid functionals. The magnetic moments are slightly increased, but the predicted dielectric constants are decreased. The valence band density of states (DOS) predicted by DD0-RSH-CAM and DSH0 are as satisfactory as HSE03 in comparison to experimental spectra, however, the conduction band DOS are shifted to higher energies by about 2 eV compared to HSE03. Self-consistent DD-RSH-CAM and DSH deteriorate the results with a significant overestimation of band gaps.