504 publications from this institution
The crystal structure and phase stability of selenium and tellurium have been investigated by total-energy calculations within the local-density approximation (LDA), without and including generalized-gradient corrections (GGC). We find that the LDA underestimates the equilibrium volume by as much as 15% and predicts a crystal structure that is much more isotropic than observed: the nearest-neighbor distances in the helical chains are increased, but the interchain distances are reduced. Adding generalized-gradient corrections to the LDA results in a crystal structure, equilibrium volume, and binding energy in better agreement with experiment. We show that the effect of the GGC's is to add a uniform internal pressure to the system, the local bonding properties at constant volume, however, are unchanged.
We propose an approach for computing the Gibbs free energy difference between phases of a material. The method is based on the determination of the average force acting on interfaces that separate the two phases of interest. This force, which depends on the Gibbs free energy difference between the phases, is computed by applying an external harmonic field that couples to a parameter which specifies the two phases. Validated first for the Lennard-Jones model, we demonstrate the flexibility, efficiency, and practical applicability of this approach by computing the melting temperatures of sodium, magnesium, aluminum, and silicon at ambient pressure using density functional theory. Excellent agreement with experiment is found for all four elements, except for silicon, for which the melting temperature is, in agreement with previous simulations, seriously underestimated.
The B3LYP hybrid functional has shown to successfully predict a wide range of molecular properties. For periodic systems, however, the failure to attain the exact homogeneous electron gas limit as well as the semiempirical construction turns out to be a major drawback of the functional. We rigorously assess the B3LYP functional for solids through calculations of lattice parameters, bulk moduli, and thermochemical properties (atomization energies and reaction energies). The theoretical lattice constants overestimate the experimental ones by approximately 1%, and hence behave similarly to the PBE gradient-corrected exchange-correlation functional. B3LYP atomization energies of solids are drastically worse than those of nonempirical hybrid Hartree-Fock/density functionals (HF/DFT) such as PBE0 and HSE03. These large errors can be traced back to the lack of a proper description of "free-electron-like" systems with a significant itinerant character (metals and small gap semiconductors). Similar calculations using the popular semiempirical B3PW91 hybrid functional, which fulfills the uniform electron gas limit, show a clear improvement over B3LYP regarding atomization energies. Finally, theoretical values for heats of formation for both the B3LYP as well as the B3PW91 functionals are presented. These document a most likely fortuitously good agreement with experiment for the B3LYP hybrid functional.
The bonding of alkali-earth, transition metal and lanthanide atoms in C60 is studied using density functional theory. In agreement with previous studies [1–3], the bonding is found to be mostly ionic, but a significant hybridisation between the metal d orbitals and carbon p orbitals is also encountered. For all metal atoms we find a similar energetic site ordering, where adsorption in the proximity of the hexagonal rings is most favourable. The energy differences between different adsorption sites however varies strongly with the metal atom. For metal atoms without d electrons, such as Ca, the potential energy surface is fairly flat, which should result in a significantly higher mobility.
Using low energy electron diffraction (LEED), density functional theory (DFT) and scanning tunneling microscopy (STM), we have re-analyzed the Pd ( 1 0 0 ) - ( 5 × 5 ) R 27 ° – O surface oxide structure consisting, in the most recent model, of a strained PdO(101) layer on top of the Pd(100) surface [M. Todorova et al., Surf. Sci. 541 (2003) 101]. Both, DFT simulations using the Vienna Ab initio Simulation Package (VASP) and tensor LEED I(V) analysis of newly acquired LEED experimental data, show that the PdO(101) model is essentially correct. However, compared to the previous study, there is a horizontal shift of the PdO(101) layer with respect to the Pd(100) substrate. The atomic coordinates derived by DFT and LEED (R P =0.162) are in excellent agreement with each other. We also present STM images with atomic resolution showing domain boundaries on the surface oxide and discuss the bonding geometry between the surface oxide and the substrate.
The correlation energy of the homogeneous electron gas is evaluated by solving the Bethe-Salpeter equation (BSE) beyond the Tamm-Dancoff approximation for the electronic polarisation propagator. The BSE is expected to improve upon the random phase approximation, owing to the inclusion of exchange diagrams. For instance, since the BSE reduces in second order to M{\o}ller-Plesset perturbation theory, it is self-interaction free in second order. Results for the correlatione energy are compared with Quantum Monte Carlo benchmarks and excellent agreement is observed. For low densities, however, we find imaginary eigenmodes in the polarisation propagator. To avoid the occurence of imaginary eigenmodes, an approximation to the BSE kernel is proposed, which allows to completely remove this issue in the low electron density region. We refer to this approximation as the random phase approximation with screened exchange (RPAsX). We show that this approximation even slightly improves upon the standard BSE kernel.
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We present lattice constants, bulk moduli, and atomization energies of solids using the correlation energy evaluated within the adiabatic connection fluctuation-dissipation framework and applying the random-phase approximation. Recently, we have shown [Phys. Rev. Lett. 103, 056401 (2009)] that geometrical properties and heats of formation are well described within this approximation. We extend this study to a larger set of materials and focus on the treatment of metals and the effect introduced by the frozen-core approximation.
