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We present ab initio quantum-mechanical molecular-dynamics calculations based on the calculation of the electronic ground state and of the Hellmann-Feynman forces in the local-density approximation at each molecular-dynamics step. This is possible using conjugate-gradient techniques for energy minimization, and predicting the wave functions for new ionic positions using subspace alignment. This approach avoids the instabilities inherent in quantum-mechanical molecular-dynamics calculations for metals based on the use of a fictitious Newtonian dynamics for the electronic degrees of freedom. This method gives perfect control of the adiabaticity and allows us to perform simulations over several picoseconds.
Exposing vanadium oxide nanoparticles on a Pd(111) surface to reducing conditions is shown to cause a spreading of the oxide over the metal until a reduced oxide phase covers the entire surface. Reoxidation reverses this process and oxide island structures and bare metal patches are reestablished. The physical origin of this wetting-dewetting process is revealed at the atomic level by in situ variable-temperature scanning tunneling microscopy and in terms of a surface oxide phase stability diagram, as calculated by density functional theory as a function of the chemical potential of oxygen and the vanadium concentration.
The growth and the atomic structure of epitaxial vanadium oxide thin films on Pd(111) have been investigated by scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED) and high-resolution electron energy loss spectroscopy (HREELS), combined with ab initio density-functional theory (DFT) calculations. At submonolayer coverage a well ordered (4×4) oxide overlayer forms, which transforms into a porous oxide network with an internal (2×2) periodicity upon exposure to H2 at room temperature. The (2×2) phase represents an interface-stabilised surface–V2O3 layer, which becomes compact upon mild annealing in vacuum exhibiting a (2×2) honeycomb structure. Between 0.5 and 1 monolayer equivalents (MLE) the growth of oxide islands with a zigzag stripe structure is observed along with the (2×2) layer. At 1 MLE two distinct VO2-like phases are coexistent at the surface in the form of islands with rectangular and hexagonal structures, interestingly they are clearly different from the known bulk-type rutile VO2 lattice. The detailed atomic structure and energetic stability of these monolayer V-oxide phases have been revealed by the DFT calculations. Above 2 MLE three-dimensional crystallites grow epitaxially on Pd(111) with the corundum structure, which is typical of the bulk-type V2O3. Two stable V2O3(0001) terminations have been found in the STM images, which are due to bulk-type oxygen planes and terminal vanadyl species, as suggested by the DFT calculations and confirmed by high-resolution electron energy loss phonon spectra.
First-principles molecular-dynamics simulations of liquid selenium at the temperatures 570, 870, and 1370 K are presented. It is shown that calculations based on the local-density approximation are not satisfactory, because they seriously overestimate the equilibrium density of the solid and the liquid, and they give only rough agreement with measured structural data when the liquid is simulated at the experimental density. The inclusion of gradient corrections mitigates these problems, and most of the results presented are based on the generalized gradient approximation. The simulations are used to investigate the three-dimensional structure of the liquid, and results are presented for the bond-angle and dihedral-angle distributions, the concentration and structure of different ${\mathrm{Se}}_{n}$ rings, and the temperature-dependent length of chains. The self-diffusion constants and vibrational spectra are shown to agree satisfactorily with experimental data, and an analysis of the lifetime of covalent bonds gives insight into the diffusion mechanism. The electronic density of states of the liquid displays all the features known for the solid, and this indicates that the electronic structure changes little on melting.
We present ab initio calculations using density-functional theory (DFT) within the local density approximation (LDA) on PbO in the \ensuremath{\alpha} phase and the idealized CsCl structure. The calculated structure of the \ensuremath{\alpha} phase, which is predicted to be the most stable, is in good agreement with experiment. It has a nonspherical electron distribution forming what could be described as sterically active lone pair formed by the ${6s}^{2}$ electrons. However, analysis of the partial density of states reveals mixing of the Pb $6s$ with the oxygen $2p$ electronic states indicating that the classical theory of hybridization of the lead $6s$ and $6p$ orbitals is incorrect and that the ``lone pair'' is the result of the lead-oxygen interaction.
We establish the energy profiles for thiophene hydrodesulfurization on the edge-(1010) surface for relevant model mechanisms. Two kinds of adsorption configurations (η 1 and η 5) for the thiophene and its hydrogenated derivatives are taken into account. We have considered a supply of hydrogen either from Mo-SH surface groups or from Mo-H groups. The η 5 configuration, which is likely to occur at low surface coverages, is clearly the most favorable one for the hydrogenation steps. In the η 1 configuration, the role of co-adsorbed -SH groups becomes crucial for the hydrogen transfer during the hydrogenation steps. At this stage of the study, the sulfur removal step appears as the most unfavorable step for a non-promoted catalyst.
The authors investigate the construction of norm-conserving 'soft-core' pseudopotentials with improved convergence properties of the plane-wave and perturbation expansions. The key factor is found to be the kinetic energy of the valence pseudo-orbitals. The total kinetic energy controls the convergence of the perturbation expansion of the total energy, the kinetic energy contained in the Fourier components beyond a certain cut-off limits the convergence of the plane-wave expansion. The simultaneous optimization of both expansions allows them to use the same pseudopotential in a rapidly convergent total-energy calculation for the crystalline phases, and in the calculation of interatomic forces to be used in atomistic simulations of the disordered phases.
The structural and cohesive properties of more than thirty transition-metal sulphides of various stoichiometries and crystal structures have been investigated using density functional theory, with the aim of establishing a correlation between the strength of the metal - sulphur bond and the catalytic activities of these materials. It is shown that the local density approximation has a tendency to overestimate the strength of the bonding. The overbinding manifests itself in the prediction of too small atomic volumes and too large cohesive energies. Non-local corrections to the local exchange - correlation functional in the form of a generalized-gradient approximation correct the overbinding (albeit with a certain tendency to overcorrect, especially for the sulphides of the heavy transition metals) and result in accurate structural prediction and cohesive energies. A correlation between the sulphur - metal bond strength and the catalytic activities is established.
