Philip J. D. Lindan

First-principles simulation, meaning density-functional theory calculations with plane waves and pseudopotentials, has become a prized technique in condensed-matter theory. Here I look at the basics of the suject, give a brief review of the theory, examining the strengths and weaknesses of its implementation, and illustrating some of the ways simulators approach problems through a small case study. I also discuss why and how modern software design methods have been used in writing a completely new modular version of the CASTEP code. 1. Overview The simulator builds a model of a real system and explores its behaviour. The model is a mathematical one and the exploration is done on a computer, and in many ways simulation studies share the same mentality as experimental ones. However, in a simulation there is absolute control and access to detail, the ability to compute almost any observable, and given enough computer muscle, exact answers for the model. These strengths have been exploited for

Using first-principles calculations we show that the adsorption of atomic hydrogen on graphene opens a substantial gap in the electronic density of states in which lies a spin-polarized gap state. This spin is quenched by the presence of a rotated C-C bond (a Stone-Wales defect) adjacent to or distant from the H atom. We explain these findings and discuss the implications for nanotubes and magnetic nanographene. Furthermore, we demonstrate that the combined effect of high curvature and a Stone-Wales defect makes H2 chemisorption close to being thermodynamically favorable.

Using first-principles density-functional methods we show that a monolayer of water on the rutile (110) surface contains ${\mathrm{H}}_{2}\mathrm{O}$ in both molecular and dissociated forms. Intermolecular hydrogen bonding stabilizes this configuration with respect to the complete dissociative adsorption which would be predicted from studies at lower coverage. The proposed mixed adsorption mode is fully consistent with experimental data, reconciles apparent conflicts within these data, and explains discrepancies between experiment and previous calculations.

The oxygen vacancy in WO(3) has previously been implicated in the electrochromism mechanism in this material. Previous theoretical calculations on the oxygen vacancy in WO(3) have not considered the full range of crystal structures adopted by the material. Here we report studies of the oxygen vacancy in seven crystal phases. The use of a very accurate tungsten plane-wave pseudopotential means that a byproduct of this study is a more detailed and complete picture of undefected WO(3) than previously available. Electronic structures of the crystal phases in both undefected and defected systems have been calculated and are discussed. The band gap in WO(3) is dependent upon bonding-antibonding interactions, these being dependent upon overlap in each direction. The effect of an oxygen vacancy is dependent upon the availability of both Op and Wd electrons, this being different for the various phases. A variety of behavior is predicted, which may be explained in terms of O2p-W5d mixing, including the formation of long W-W dimer bonds. It is found that the nature of a polaron in this material is dependent upon both the crystal structure and distribution of oxygen vacancies.

We have performed plane-wave pseudopotential density-functional theory calculations on the stoichiometric and reduced ${\mathrm{TiO}}_{2}$ (110) surface, the 2\ifmmode\times\else\texttimes\fi{}1 and 1\ifmmode\times\else\texttimes\fi{}2 reconstructions of the surface formed by the removal of bridging-oxygen atoms, and on the oxygen vacancy in the bulk. The effect of including spin polarization is investigated, and it is found to give a qualitatively different electronic structure compared with a spin-paired description. In the spin-polarized solutions, the excess electrons generated by oxygen reduction occupy localized band-gap states formed from Ti (3d) orbitals, in agreement with experimental findings. In addition, the inclusion of spin polarization substantially lowers the energy of all the systems studied, when compared with spin-paired solutions. However, spin-polarization does not change the relative stability of the two reconstructions, which remain energetically equivalent.