Publications

The formal relationship between ultrasoft (US) Vanderbilt-type pseudopotentials and Bl\"ochl's projector augmented wave (PAW) method is derived. It is shown that the total energy functional for US pseudopotentials can be obtained by linearization of two terms in a slightly modified PAW total energy functional. The Hamilton operator, the forces, and the stress tensor are derived for this modified PAW functional. A simple way to implement the PAW method in existing plane-wave codes supporting US pseudopotentials is pointed out. In addition, critical tests are presented to compare the accuracy and efficiency of the PAW and the US pseudopotential method with relaxed core all electron methods. These tests include small molecules $({\mathrm{H}}_{2}{,\mathrm{}\mathrm{H}}_{2}{\mathrm{O},\mathrm{}\mathrm{Li}}_{2}{,\mathrm{}\mathrm{N}}_{2}{,\mathrm{}\mathrm{F}}_{2}{,\mathrm{}\mathrm{BF}}_{3}{,\mathrm{}\mathrm{SiF}}_{4})$ and several bulk systems (diamond, Si, V, Li, Ca, ${\mathrm{CaF}}_{2},$ Fe, Co, Ni). Particular attention is paid to the bulk properties and magnetic energies of Fe, Co, and Ni.

We have used density-functional theory and the nudged elastic-band method to calculate migration pathways and estimated the activation energy for the diffusion of oxygen vacancies in \ensuremath{\alpha}-quartz. While the energy barrier for the diffusion of a neutral vacancy is very high, 4.1 eV, the binding of a triplet-state exciton to the vacancy lowers the barrier to 1.7 eV and the attachment of a hole lowers the barrier to 1.9 eV, making the vacancy mobile at commonly used annealing temperatures.

We present an implementation and analysis of a stochastic high performance algorithm to calculate the correlation energy of three-dimensional periodic systems in second-order Møller-Plesset perturbation theory (MP2). In particular we measure the scaling behavior of the sample variance and probe whether this stochastic approach is competitive if accuracies well below 1 meV per valence orbital are required, as it is necessary for calculations of adsorption, binding, or surface energies. The algorithm is based on the Laplace transformed MP2 (LTMP2) formulation in the plane wave basis. The time-dependent Hartree-Fock orbitals, appearing in the LTMP2 formulation, are stochastically rotated in the occupied and unoccupied Hilbert space. This avoids a full summation over all combinations of occupied and unoccupied orbitals, as inspired by the work of Neuhauser, Rabani, and Baer [J. Chem. Theory Comput. 9, 24 (2013)]. Additionally, correlated sampling is introduced, accelerating the statistical convergence significantly.

We show that two all-electron methods, the full-potential linearized augmented wave method and the projector augmented wave method, yield a metallic groundstate for ${\mathrm{Ce}}_{2}{\mathrm{O}}_{3}$, as opposed to the experimentally observed insulating groundstate. This is in contrast to previous pseudopotential calculations [Phys. Rev. B 71, 041102(R) (2005)], which suffered from an erroneous overestimation of the spin-enhancement factor and a resulting overestimation of the exchange splitting.

Ab initio MD studies of the structural and electronic properties of K–Te alloys at three different stoichiometric compositions are presented. The tellurium-rich mixture K0.12Te0.88 is entirely determined by the Te atoms, with a structure and electronic properties that are very similar to those of pure liquid Te. The properties of equiatomic KTe at 770 K are determined by the strong iono-covalent bonding, but the order of the crystalline phase, consisting of Te2 2− dumbbells in a matrix of K cations, is blurred to a very high degree. The strong coupling of the potassium cations to the Te system is also shown by the diffusion constant. In the octet compound K2Te at 1300 K we found that the ionic character of the solid salt is preserved in the liquid. The electronic properties of both liquid mixtures, although modified by thermal disorder, show some similarities to the crystalline phases, especially a deep minimum at the Fermi edge, confirming the low conductivity values that are experimentally measured.

It has recently become possible to calculate the free energy and other thermodynamic functions of solids and liquids using density functional theory to treat the quantum mechanics of the electrons. We present the main ideas that have made this possible, emphasizing the key role of thermodynamic integration and the importance of well-adapted reference systems in the computation of the free energy. We show how ab initio methods have been used to calculate the melting and other phase-equilibrium properties of simple materials, and the thermal-equilibrium concentrations of point defects in crystals. We point out the possibility of adapting techniques that are already available in order to calculate chemical potentials, solubilities, equilibrium constants, and other thermodynamic functions that are important in physical chemistry. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 871–879, 2000

We use the data generated in recent first-principles molecular-dynamics simulations of liquid selenium at the temperatures 570, 870, and 1370 K to investigate defects in the liquid. The defects represent disruptions of the chain structure associated with onefold and threefold atoms ${(C}_{1}$ and ${C}_{3}$ defects). We stress that for a full understanding we need to analyze the defects from the viewpoints of atomic coordination, defect dynamics, and electronic structure, and we develop analysis techniques to do this. We find that localized electronic states at the top of the valence band and the bottom of the conduction band are associated with ${C}_{1}$ and ${C}_{3}$ defects, respectively. At 570 K, the concentration of defects is very low, and they exist as ${C}_{1}$ and ${C}_{3}$ defects in bound pairs (intimate valence-alternation pairs), but at high temperatures the defects are isolated and are mainly of ${C}_{1}$ type. The defect concentrations are used to determine the mean length of Se chains, which we find to be much smaller than the values deduced from NMR measurements at lower temperatures, but in reasonable agreement at 1370 K. Analysis of the defect dynamics shows that the residence time of defects on individual atoms becomes extremely short---comparable with the vibrational period---at high temperature.

Spin-polarized calculations in solids have generally been confined to a global quantization axis to simplify both the theoretical model and its implementation in self-consistent codes. This approximation is justified as many materials exhibit a collinear magnetic order. However, in recent years much interest has been directed towards noncollinear magnetism in which the magnetization density is a continuous vector variable of position. In this paper we develop the all-electron projector augmented-wave (PAW) method for noncollinear magnetic structures, based on a generalized local-spin-density theory. The method allows both the atomic and magnetic structures to relax simultaneously and self-consistently. The algorithms have been implemented within a powerful package called VASP (Vienna ab initio simulation package), which has been used successfully for a large variety of different systems such as crystalline and amorphous semiconductors, simple liquids, and transition metals. The approach has been used to study small clusters of Fe and Cr; some of these clusters show noncollinear magnetic arrangements.

Density functional theory is used to study carbon nanotubes grown inside a channel-like environment. Two types of systems were investigated: the zigzag nanotubes inside AlPO4-5 zeolite (AFI) channels and the achiral double-walled nanotubes. In this work the energetical stability of the selected systems is evaluated and the shifting and splitting of the phonons, especially the radial-breathing mode is analyzed in a more detail.