Publications

We present an overview of the description of structural, thermochemical, and electronic properties of extended systems using several well known hybrid Hartree–Fock/density-functional-theory functionals (PBE0, HSE03, and B3LYP). In addition we address a few aspects of the evaluation of the Hartree–Fock exchange interactions in reciprocal space, relevant to all methods that employ a plane wave basis set and periodic boundary conditions.

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.

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.

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.

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.

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.

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.

No abstract is provided for this article.

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.

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.