Publications

Ultrasoft pseudopotentials applied to magnetic Fe, Co, and Ni: From atoms to solids

We present a study of the accuracy, transferability, and plane-wave convergence properties of ultrasoft Vanderbilt-type pseudopotentials for Fe, Co, and Ni in the context of atomic, molecular, and solid calculations. Special attention has been given to the magnetic properties of these systems. To go beyond the local-spin-density-approximation, generalized gradient approximations for the exchange-correlation functional have been included. All calculations have been performed using a plane-wave basis set, and we show that ultrasoft pseudopotentials allow -- as expected -- for a considerably lower cutoff energy than standard soft norm-conserving pseudopotentials. Lattice properties show very good agreement with all-electron calculations and experiment, while larger discrepancies exist for magnetic structural energy differences (which however remain smaller than 2 mRy/atom). These differences can be traced back to the frozen core approximation which is implicitly assumed in the construction of the pseudopotentials. More accurate results for the magnetization energies of atomic configurations can be obtained by treating the $3p$ semicore states as valence states.

High Pressure Polymorphism in Silica

Fundamental crystal chemistry and first-principles total-energy calculations are used to examine stable and metastable high-pressure silica structures. We find that a large class of energetically competitive phases can be generated from hcp arrays of oxygen with silicon occupying one-half of the octahedral sites. Calculations for specific structures provide an explanation for a number of recent high-pressure results for crystalline silica and allow us to understand the nature of the short- and intermediate-range order in the high-pressure amorphous state.

Novel Stabilization Mechanism on Polar Surfaces: ZnO(0001)-Zn

The (1x1) terminated (0001)-Zn surface of wurtzite ZnO was investigated with scanning tunneling microscopy. The surface is characterized by the presence of nanosized islands with a size-dependent shape and triangular holes with single-height, O-terminated step edges. It is proposed that the resulting overall decrease of the surface Zn concentration stabilizes this polar surface. Ab initio calculations of test geometries predict triangularly shaped reconstructions over a wide range of oxygen and hydrogen chemical potentials. The formation of these reconstructions appears to be electrostatically driven.

$GW$ vertex corrected calculations for molecular systems

Hedin's scheme is solved with the inclusion of the vertex function ($GW\Gamma$) for a set of small molecules. The computational scheme allows for the consistent inclusion of the vertex both at the polarizability level and in the self-energy. A diagrammatic analysis shows that the self-energy formed with this four-point vertex does not lead to double counting of diagrams, that can be classified as direct "bubbles" and exchange diagrams. By removing the exchange diagrams from the self-energy, a simpler approximation is obtained, called $GW^{\rm{tc-tc}}$. Very good agreement with expensive wavefunction-based methods is obtained for both approximations.

$GW$100: a plane wave perspective for small molecules

In a recent work, van Setten and coworkers have presented a carefully converged $G_0W_0$ study of 100 closed shell molecules [J. Chem. Theory Comput. 11, 5665 (2015)]. For two different codes they found excellent agreement to within few 10 meV if identical Gaussian basis sets were used. We inspect the same set of molecules using the projector augmented wave method and the Vienna ab initio simulation package (VASP). For the ionization potential, the basis set extrapolated plane wave results agree very well with the Gaussian basis sets, often reaching better than 50 meV agreement. In order to achieve this agreement, we correct for finite basis set errors as well as errors introduced by periodically repeated images. For electron affinities below the vacuum level differences between Gaussian basis sets and VASP are slightly larger. We attribute this to larger basis set extrapolation errors for the Gaussian basis sets. For quasi particle (QP) resonances above the vacuum level, differences between VASP and Gaussian basis sets are, however, found to be substantial. This is tentatively explained by insufficient basis set convergence of the Gaussian type orbital calculations as exemplified for selected test cases.

Structural, vibrational and quasiparticle properties of the Peierls semiconductor $\rm BaBiO_3$: a hybrid functional and self-consistent GW+vertex-corrections study

$\rm BaBiO_3$ 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.

High-resolution core-level spectroscopy study of the ultrathin aluminum oxide film on NiAl(110)

We have studied the ultrathin aluminum oxide film on NiAl(110) by a combination of high-resolution core-level spectroscopy and density functional theory calculations. Energy-dependent core-level data from the O 1$s$ and Al 2$p$ levels allows for a distinction between oxygen and aluminum atoms residing at the surface or inside the aluminum oxide film. A comparison to calculated core-level binding energies from the recent model by Kresse et al. [Science 308, 1440 (2005)] reveals good agreement with experiment, and the complex spectroscopic signature of the thin Al oxide on NiAl(110) can be explained. Our assignment of a shifted component in the O $1s$ spectra to oxygen atoms at the surface with a particular Al and oxygen coordination may have implications for the interpretation of photoelectron-diffraction experiments from similar ultrathin aluminum oxide films.

<b><i>Ab initio</i></b>study of the (0001) surfaces of hematite and chromia: Influence of strong electronic correlations

We present a detailed ab initio investigation of the stability, the structural, electronic, and magnetic properties of the (0001) surfaces of hematite $({\mathrm{Fe}}_{2}{\mathrm{O}}_{3})$ and chromia or eskolaite $({\mathrm{Cr}}_{2}{\mathrm{O}}_{3})$. Strong electron correlation effects not included in a density-functional description are described by a Hubbard-type on-site Coulomb repulsion (the $\mathrm{DFT}+U$ approach). For bulk chromia we find, complementing our recent work on hematite [Rollmann et al., Phys. Rev. B 69, 165107 (2004)] that the inclusion of correlation effects leads to an improved description of the structural, electronic, and magnetic properties. In particular, the increased exchange splitting of the $d$ band changes the character of the insulating gap from a pure $d\text{\ensuremath{-}}d$ Mott-Hubbard type to intermediate between $d\text{\ensuremath{-}}d$ and charge-transfer insulator. For both oxides, the strong correlation effects have a dramatic influence on the surface stability: oxygen-terminated surfaces are strongly disfavored because of the increased energetic cost of stabilizing a higher oxidation state of the transition metal close to the surface. The stability of metal-terminated surfaces even under oxidizing conditions agrees with the most recent STM and LEED data. For ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}(0001)$ where detailed experimental information on the surface structure is available, quantitative agreement of the calculated surface relaxations is achieved. Detailed results on the surface electronic structure (valence-band spectra and core-level shifts) and the surface magnetic properties are presented.

Making the random phase approximation to electronic correlation accurate

We show that the inclusion of second-order screened exchange to the random phase approximation allows for an accurate description of electronic correlation in atoms and solids clearly surpassing the random phase approximation, but not yet approaching chemical accuracy. From a fundamental point of view, the method is self-correlation free for one-electron systems. From a practical point of view, the approach yields correlation energies for atoms, as well as for the jellium electron gas within a few kcal/mol of exact values, atomization energies within typically 2-3 kcal/mol of experiment, and excellent lattice constants for ionic and covalently bonded solids (0.2% error). The computational complexity is only O(N(5)), comparable to canonical second-order Møller-Plesset perturbation theory, which should allow for routine calculations on many systems.

Ab Initio Molecular Dynamical Simulation on H2 Adsorption and Storage in Carbon-Based Materials

Ab initio molecular dynamics simulations have been utilized to study hydrogen adsorption and storage in carbon-based materials. The method was first applied to studies of H2 adsorption in potassium-intercalated graphite of the second stage. The calculated results were in excellent agreement with the experimental observations. We subsequently performed calculations for H2 adsorption in single walled carbon nanotubes (SWNT’s). We show that SWNT’s undergo significant structural deformation at various temperatures and the curved carbons are responsible for the strong C-H2 interaction.

Maximally localized Wannier functions in LaMnO3 within PBE+U, hybrid functionals, and GW: an efficient route to construct ab-initio tight-binding parameters for e_g perovskites

Using the newly developed VASP2WANNIER90 interface we have constructed maximally localized Wannier functions (MLWFs) for the e_g states of the prototypical Jahn-Teller magnetic perovskite LaMnO3 at different levels of approximation for the exchange-correlation kernel. These include conventional density functional theory (DFT) with and without additional on-site Hubbard U term, hybrid-DFT, and single shot GW. By suitably mapping the MLWFs onto an effective e_g tight-binding (TB) Hamiltonian we have computed a complete set of TB parameters which should serve as guidance for more elaborate treatments of correlation effects in effective Hamiltonian-based approaches. The method-dependent changes of the calculated TB parameters and their interplay with the electron-electron (el-el) interaction term are discussed and interpreted. We discuss two alternative model parameterizations: one in which the effects of the el-el interaction are implicitly incorporated in the otherwise noninteracting TB parameters, and a second where we include an explicit mean-field el-el interaction term in the TB Hamiltonian. Both models yield a set of tabulated TB parameters which provide the band dispersion in excellent agreement with the underlying ab initio and MLWF bands.

Electron–phonon coupling in semiconductors within the GW approximation

The magnitude of the renormalization of the band gaps due to zero-point motions of the lattice is calculated for 18 semiconductors, including diamond and silicon. This particular collection of semiconductors constitute a wide range of band gaps and atomic masses. The renormalized electronic structures are obtained using stochastic methods to sample the displacement related to the vibrations in the lattice. Specifically, a recently developed one-shot method is utilized where only a single calculation is needed to get similar results as the one obtained by standard Monte-Carlo sampling. In addition, a fast real-space GW method is employed and the effects of G0W0 corrections on the renormalization are also investigated. We find that the band-gap renormalizations inversely depend on the mass of the constituting ions, and that for the majority of investigated compounds the G0W0 corrections to the renormalization are very small and thus not significant.

Theoretical Analysis of Oxygen Diffusion in monoclinic HfO2

In this report, we present the detailed analysis of the interstitial oxygen (O2+, O0, O2-) diffusion in monoclinic HfO2 (hafnia) using the first principles calculations. The interstitial oxygen atom kicks out the oxygen atom at the 3-fold-site and occupies the 3-fold-site. And then the newly kicked-out interstitial oxygen atom jumps to the nearest neighbor site and couples again with the atoms at the crystal sites. This kick-out- mechanism is valid for all charge states of the interstitial oxygen in monoclinic HfO2. In hafnia, the interstitial oxygen atom can take 3 charge states (+2, 0, -2) depending on the chemical potential (Ef), whereas the oxygen-vacancy in hafnia can get +2 or 0 charge state being dependent on Ef. In the lower range of Ef, O2+ and O0 might contribute. In the middle range of Ef, the O2- does not contribute to the diffusion process in hafnia because of the pair annihilation process between O2- and oxygen vacancy (V2+) defect pair. We can simulate such a pair annihilation process in hafnia. In the higher range, O2- might contribute the diffusion process.

Optical spectra of Si nanocrystallites: Bethe-Salpeter approach versus time-dependent density-functional theory

Two state-of-the-art approaches based on the quasiparticle-Bethe-Salpeter equation (QP-BSE) and time-dependent density-functional theory (TDDFT) for different functionals are applied to calculate optical-absorption spectra of Si nanocrystallites passivated with hydrogen. All-electron wave functions are generated within the projector-augmented wave method. The results of the two many-body approaches are used to discuss the interplay of quasiparticle, local-field (LF), and excitonic effects. The QP approach gives rise to blueshifts of the absorption spectra, whereas the LF effects and electron-hole exchange redistribute the oscillator strengths toward higher energies. The screened electron-hole attraction leads to slightly larger optical gaps than the ones found for independent particles described within the local-density approximation (LDA) for exchange and correlation (XC). The results within the TDDFT using the LDA kernel confirm the influence of LF effects. When a hybrid functional for XC is used, the TDDFT spectra show the same tendencies as the QP-BSE ones but still indicate a reduced electron-hole attraction. An effective-medium theory is used to examine the role of local fields due to the nanocrystal arrangement.

Computing Nonradiative Capture Coefficients from First Principles

Comment on “Magnetism of the V(001) surface: Contradictory results from pseudopotential and linearized augmented plane-wave calculations”

Received 8 November 2001DOI:https://doi.org/10.1103/PhysRevB.66.146401©2002 American Physical Society

Hydrogen adsorption on an open surface: H_2/Pd(210)

Hybrid functionals applied to extended systems

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.