Publications

Benchmark Phaseless Auxiliary-Field Quantum Monte Carlo Method for Small Molecules

We report a scalable Fortran implementation of the phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC) and demonstrate its excellent performance and beneficial scaling with respect to system size. Furthermore, we investigate modifications of the phaseless approximation that can help to reduce the overcorrelation problems common to the ph-AFQMC. We apply the method to the 26 molecules in the HEAT set, the benzene molecule, and water clusters. We observe a mean absolute deviation of the total energy of 1.15 kcal/mol for the molecules in the HEAT set; close to chemical accuracy. For the benzene molecule, the modified algorithm despite using a single-Slater-determinant trial wavefunction yields the same accuracy as the original phaseless scheme with 400 Slater determinants. Despite these improvements, we find systematic errors for the CN, CO$_2$, and O$_2$ molecules that need to be addressed with more accurate trial wavefunctions. For water clusters, we find that the ph-AFQMC yields excellent binding energies that differ from CCSD(T) by typically less than 0.5 kcal/mol.

RPA natural orbitals and their application to post-Hartree-Fock electronic structure methods

We present a method to approximate post-Hartree-Fock correlation energies by using approximate natural orbitals obtained by the random phase approximation (RPA). We demonstrate the method by applying it to the helium atom, the hydrogen and fluorine molecule, and to diamond as an example of a periodic system. For these benchmark systems, we show that RPA natural orbitals converge the MP2 correlation energy rapidly. Additionally, we calculated full configuration interaction energies for He and H2, which are in excellent agreement with the literature and experimental values. We conclude that the proposed method may serve as a compromise to reach good approximations to correlation energies at moderate computational cost, and we expect the method to be especially useful for theoretical studies on surface chemistry by providing an efficient basis to correlated wave function based methods.

Layer-by-layer phase transformation in Ti3O5 revealed by machine-learning molecular dynamics simulations

Reconstructive phase transitions involving breaking and reconstruction of primary chemical bonds are ubiquitous and important for many technological applications. In contrast to displacive phase transitions, the dynamics of reconstructive phase transitions are usually slow due to the large energy barrier. Nevertheless, the reconstructive phase transformation from β - to λ -Ti 3 O 5 exhibits an ultrafast and reversible behavior. Despite extensive studies, the underlying microscopic mechanism remains unclear. Here, we discover a kinetically favorable in-plane nucleated layer-by-layer transformation mechanism through metadynamics and large-scale molecular dynamics simulations. This is enabled by developing an efficient machine learning potential with near first-principles accuracy through an on-the-fly active learning method and an advanced sampling technique. Our results reveal that the β − λ phase transformation initiates with the formation of two-dimensional nuclei in the a b -plane and then proceeds layer-by-layer through a multistep barrier-lowering kinetic process via intermediate metastable phases. Our work not only provides important insight into the ultrafast and reversible nature of the β − λ transition, but also presents useful strategies and methods for tackling other complex structural phase transitions.

A theoretical study of the H-induced reconstructions of the Pd(110) surface

It is now established by experiments that the chemisorption of hydrogen on Pd(110) surface can induce reconstructions. Under different conditions of coverage and temperature, two types of reconstruction have been found: pairing-row and missing-row. Although the way in which the substrate reconstructs can be clearly deduced from experiments, the way in which H atoms are arranged on the reconstructed surfaces is not so well established. This is due to the difficulty of directly “seeing” H atoms by experimental methods. In this work, we report a systematic study of the reconstructions on Pd(110) by ab initio calculations based on density functional theory with local density approximation (LDA), generalized gradient approximation (GGA) and ultrasoft pseudopotentials. A variety of models are studied in detail. We show that only particular arrangements of H atoms can induce the surface reconstructions considered. The driving force for the reconstructions is analysed.

Donor defects and small polarons on the TiO2(110) surface

The role of defects in the chemical activity of the rutile TiO2(110) surface remains a rich topic of research, despite the rutile (110) being one of the most studied surfaces of transition-metal oxides. Here, we present results from hybrid functional calculations that reconcile apparently disparate views on the impact of donor defects, such as oxygen vacancies and hydrogen impurities, on the electronic structure of the (110) rutile surface. We find that the bridging oxygen vacancy and adsorbed or substitutional hydrogen are actually shallow donors, which do not induce gap states. The excess electrons from these donor centers tend to localize in the form of small polarons, which are the factual cause of the deep states ∼1 eV below the conduction band, often observed in photoelectron spectroscopy measurements. Our results offer a new framework for understanding the surface electronic structure of TiO2 and related oxides.

Approaching the basis-set limit of the dRPA correlation energy with explicitly correlated and Projector Augmented-wave methods

The direct random-phase approximation (dRPA) is used to calculate and compare atomization energies for the HEAT set and 10 selected molecules of the G2-1 set using both plane waves and Gaussian-type orbitals. We describe detailed procedures to obtain highly accurate and well converged results for the projector augmented-wave (PAW) method as implemented in the Vienna Ab-initio Simulation Package (VASP) as well as the explicitly correlated dRPA-F12 method as implemented in the TURBOMOLE package. The two approaches agree within chemical accuracy (1 kcal/mol) for the atomization energies of all considered molecules, both for the exact exchange as well as for the dRPA. The root mean-square deviation is 0.41 kcal/mol for the exact exchange (evaluated using density functional theory orbitals) and 0.33 kcal/mol for exact exchange plus the random-phase approximation.

Erratum to “Atomic and electronic structure of diamond (111) surfaces. I. Reconstruction and hydrogen-induced deconstruction of the one dangling-bond surface” [Surf. Sci. 366 (1996) 445]

<i>Ab initio</i>molecular dynamics for open-shell transition metals

We show that quantum-mechanical molecular-dynamics simulations in a finite-temperature local-density approximation based on the calculation of the electronic ground state and of the Hellmann-Feynman forces after each time step are feasible for liquid noble and transition metals. This is possible with the use of Vanderbilt-type ``ultrasoft'' pseudopotentials and efficient conjugate-gradient techniques for the determination of the electronic ground state. Results for liquid copper and vanadium are presented.

Reactivity of Noble Gases under High Pressure

No abstract is provided for this article.

Density functional theory study of MnO by a hybrid functional approach

The ground state properties of MnO are investigated using the plane wave based projector augmented wave technique and the so-called ''parameter-free'' hybrid functional approach PBE0 for the approximation of the exchange-correlation energy and potential. The insulating, antiferromagnetically ordered and rhombohedrally distorted B1 structure is found to be the most stable phase of MnO, consistent with experiment. The band gap of 4.02 eV, spin magnetic moment of $4.52\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$, optimized lattice parameter $a=4.40\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, rhombohedral distortion angle $\ensuremath{\alpha}={0.88}^{0}$, density of states, and magnetic properties are all in good agreement with experiment. Results obtained from standard methods such as generalized gradient approximation (GGA), $\mathrm{GGA}+\mathrm{U}$ and periodic Hartee-Fock are also reported for comparative purposes. In line with previous studies, our results suggest that the applied hybrid functional method PBE0, which combines 25% of the exact exchange with a generalized-gradient approximation, corrects the deficiency of semilocal density functionals and provides an accurate quantitative description of the structural, electronic, and magnetic properties of MnO without any adjustable parameter.

Significance of single-electron energies for the description of CO on Pt(111)

Semilocal density functionals predict that the stable adsorption site of carbon monoxide (CO) on Pt(111) is the hollow fcc site, in disagreement with experimental studies which indicate that CO adsorbs on the top site at low coverage. This site preference depends on a subtle balance between the interaction of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) with the metal substrate. Local and semilocal functionals seem to overestimate the interaction of the LUMO with the metal substrate, in turn favoring the wrong site. It is argued that this error is related to the ``gap'' problem of present density functionals and might be cured by functionals that increase the HOMO-LUMO separation.

Gaussian charge-transfer charge distributions for non-self-consistent electronic structure calculations

We present a scheme to construct charge densities for non-self-consistent density functional theory calculations. Effects of charge transfer are added onto the density of overlapping atomic charge densities by means of charge-neutral spherical atom-centered Gaussian charge distributions that have been fitted to the self-consistent charge density in amorphous model systems. The electronic structure and forces obtained from non-self-consistent calculations using these a priori constructed charge densities are in good agreement with fully self-consistent calculations, and the Gaussian charge-transfer charges exhibit a satisfactory transferability between compounds of similar stochiometry but different geometry.

First-principles calculations for VxOy grown on Pd(111)

An approach to access the stability of oxides growing on top of a metal support is presented. In combination with first-principles calculations, it allows to predict the stable structures as a function of the thickness of the evaporated metal ad-layer and as a function of the oxygen pressure. The ideas are applied to thin vanadium oxide films growing on Pd(111). To investigate the stability of these oxide films, first-principles calculations for more than 50 thin films of V x O y on Pd were performed at varying stoichiometry and coverage. The general principles determining the growth of thin vanadium oxide films on Pd(111) are discussed, and the experimental results are interpreted in the light of the first-principles calculations. At 1 ML vanadium coverage, a complicated succession of structures is predicted by the calculations. At high oxygen pressure bulk like V2O3 phases are stable. At lower oxygen pressure, however, a surface stabilised (2×2) reconstruction with a formal stoichiometry of V2O3 is predicted, and rectangular and hexagonal vanadium-dioxide phases are expected to grow. At very low oxygen pressures, first the vanadium-dioxide phases and then the surface V2O3 phase decompose and the liberated V atoms move subsurface. These predictions are in good general agreement with experiment. An important result of the study is that the metal surface stabilises thin films which have no equivalent bulk phases.

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

Correction: New insights into the 1D carbon chain through the RPA

Correction for 'New insights into the 1D carbon chain through the RPA' by Benjamin Ramberger et al., Phys. Chem. Chem. Phys., 2021, 23, 5254-5260, https://doi.org/10.1039/D0CP06607A.

Efficient iterative schemes for<i>ab initio</i>total-energy calculations using a plane-wave basis set

We present an efficient scheme for calculating the Kohn-Sham ground state of metallic systems using pseudopotentials and a plane-wave basis set. In the first part the application of Pulay's DIIS method (direct inversion in the iterative subspace) to the iterative diagonalization of large matrices will be discussed. Our approach is stable, reliable, and minimizes the number of order ${\mathit{N}}_{\mathrm{atoms}}^{3}$ operations. In the second part, we will discuss an efficient mixing scheme also based on Pulay's scheme. A special ``metric'' and a special ``preconditioning'' optimized for a plane-wave basis set will be introduced. Scaling of the method will be discussed in detail for non-self-consistent and self-consistent calculations. It will be shown that the number of iterations required to obtain a specific precision is almost independent of the system size. Altogether an order ${\mathit{N}}_{\mathrm{atoms}}^{2}$ scaling is found for systems containing up to 1000 electrons. If we take into account that the number of k points can be decreased linearly with the system size, the overall scaling can approach ${\mathit{N}}_{\mathrm{atoms}}$. We have implemented these algorithms within a powerful package called VASP (Vienna ab initio simulation package). The program and the techniques have been used successfully for a large number of different systems (liquid and amorphous semiconductors, liquid simple and transition metals, metallic and semiconducting surfaces, phonons in simple metals, transition metals, and semiconductors) and turned out to be very reliable. \textcopyright{} 1996 The American Physical Society.

The oxidation of Rh(111)

Lack of surface oxide layers and facile bulk oxide formation on Pd(110)

The oxidation of the Pd(110) surface has been studied from ultrahigh vacuum up to atmospherical pressures by combining scanning tunneling microscopy, low-energy electron diffraction and high-resolution core-level spectroscopy with in situ surface x-ray diffraction, and density-functional theory calculations. Under in situ conditions, we observe a $c(2\ifmmode\times\else\texttimes\fi{}4)$ structure which transforms via the formation of antiphase domain boundaries to a ``complex'' structure with increasing partial oxygen pressure. Contrary to other closed packed and vicinal Pd surfaces investigated so far, no surface oxide is formed, which allows for the formation of the PdO bulk oxide closer to the thermodynamic limit at temperatures relevant for catalysis.