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.

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 H 2 chemisorption close to being thermodynamically favorable. DOI: 10.1103/PhysRevLett.92.225502 PACS numbers: 61.46.+w, 71.15.Nc, 73.22.Dj Carbon is unique in possessing allotropes of each possible dimensionality, from 3D diamond to 0D fullerenes [1]. This is because the three sp hybridizations of carbon have an energy cost readily offset or exceeded by strong covalent bonding. The two recently discovered forms, C 60 and most particularly carbon nanotubes, have aroused more interest than almost any other material. This is because of the special and unique properties associated with the structures and the prospect of tuning these properties for a purpose by nanoscale manipulation of structure. The desire to apply this knowledge within nanotechnology, where nanotubes, in particular, have been earmarked as key components, is a strong additional driving force.The electronic structure of graphene, a single planar sheet of sp 2 -bonded carbon atoms, is well understood and provides the basis for the elegant theoretical predictions of the electronic structure of single-walled carbon nanotubes (SWNT) made soon after their discovery [2]. These predictions are receiving backing through very recent experiments [3]. SWNT can be metallic or semiconducting, the latter with either a small or moderate gap, depending on the way in which their structures map onto the imaginary process of rolling up a graphene sheet, with refinements due to curvature effects [3]. However, this theoretical understanding is lost once intrinsic defects [4] are present in the graphene sheet. Defects matter for several reasons: Although they in general have high formation energies the synthesis routes used to make nanotubes do not have a high yield of defect-free specimens; defects undoubtedly alter the electronic structure and therefore chemical, optical, and other properties; and finally understanding scanning-tunneling microscopy images of sp 2 -bonded carbon is difficult and there is unresolved debate over how defects manifest themselves [5]. A parallel concern is the chemical reactivity of carbon nanostructures and structure-property relationships therein. At the nanoscale electronic structure calculations become invaluable in understanding materials, and indeed the combination of theory and simulations has led the science of carbon nanotubes. Calculations on the electronic structure changes due to ring defects have appeared recently [6,7], as have some calculations on adsorption processes [8][9][10]. Our focus is on the interplay of d...

Using first-principles density-functional methods we show that a monolayer of water on the rutile (110) surface contains H 2 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. [S0031-9007(97)05099-0]

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 TiO 2 ͑110͒ surface, the 2ϫ1 and 1ϫ2 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. ͓S0163-1829͑97͒02724-0͔

The adsorption of water on the TiO2(110) surface has become the model process in efforts to understand metal oxide–aqueous solution interfaces. Considerable progress has been made in understanding low-coverage situations where first-principle calculations have been employed to good effect. However, current theory is less well developed for coverage above one monolayer. Here we present results of calculations on the adsorption process in forming the second water layer, that is, the adsorption of water on the fully hydrated surface. We show that there are many competitive adsorption sites owing to the numerous hydrogen-bonding possibilities. The presence of the second layer water molecules facilitates proton transfer among the adsorbates within chainlike configurations, and we present some illustration of these processes. We show how the adsorption energetics computed here along with recent calculations on defective surface and low-coverage adsorption may be used to provide a satisfactory interpretation of the temperature programmed desorption data for this system. Finally, we compute the vibrational spectrum for H and compare with the high-resolution electron-energy-loss spectroscopy measurements.

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