Abstract. The CASTEP code for first principles electronic structure calculations will be described. A brief, nontechnical overview will be given and some of the features and capabilities highlighted. Some features which are unique to CASTEP will be described and near-future development plans outlined.

The widespread popularity of density functional theory has given rise to an extensive range of dedicated codes for predicting molecular and crystalline properties. However, each code implements the formalism in a different way, raising questions about the reproducibility of such predictions. We report the results of a community-wide effort that compared 15 solid-state codes, using 40 different potentials or basis set types, to assess the quality of the Perdew-Burke-Ernzerhof equations of state for 71 elemental crystals. We conclude that predictions from recent codes and pseudopotentials agree very well, with pairwise differences that are comparable to those between different high-precision experiments. Older methods, however, have less precise agreement. Our benchmark provides a framework for users and developers to document the precision of new applications and methodological improvements

Additional information: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The application of variational density functional perturbation theory ͑DFPT͒ to lattice dynamics and dielectric properties is discussed within the plane-wave pseudopotential formalism. We derive a method to calculate the linear response of the exchange-correlation potential in the GGA at arbitrary wavevector. We introduce an efficient self-consistent solver based on all-bands conjugate-gradient minimization of the second order energy, and compare the performance of preconditioning schemes. Lattice-dynamical and electronic structure consequences of space-group symmetry are described, particularly their use in reducing the computational effort required. We discuss the implementation in the CASTEP DFT modeling code, and how DFPT calculations may be efficiently performed on parallel computers. We present results on the lattice dynamics and dielectric properties of ␣-quartz, the hydrogen bonded crystal NaHF 2 and the liquid-crystal-forming molecule 5CB. Excellent agreement is found between theory and experiment within the GGA.

Monte Carlo computer simulation has been used to study water confined between the layers of 2:1 clay minerals. The model systems are based on natural Mg and Na smectites. The simulation cells contain one clay layer, 64 water molecules and four magnesium or eight sodium interlayer cations. These atoms and molecules interact with each other through a new set of effective pair potentials, which we discuss. The calculations are conducted in constant (N,p,T) ensembles, at T=300 K and with a uniaxial pressure, p, of 1 M Pa applied normal to the clay sheets. All the molecules, including the clay sheets, are therefore allowed to move during the simulations. The calculated equilibrium layer spacing is 14.7±0.1 Å with interlayer Mg2+ and 14.2±0.1 Å with interlayer Na+. These spacings compare with experimental values of 15.1 Å and 14.5 Å, measured for Mg and Na saturated Chambers montmorillonite, at 79% relative humidity. The corresponding densities and average potential energies of the interlayer water molecules are 1.38±0.04 g cm−3 and −17.63±0.02 kcal mol−1, respectively, for Mg smectite and 1.14±0.04 g cm−3 and −11.77±0.02 kcal mol−1, respectively, for Na smectite. We analyze and compare the interlayer structures in the two systems.

No abstract

Density functional theory (DFT) has been used in many fields of the physical sciences, but none so successfully as in the solid state. From its origins in condensed matter physics, it has expanded into materials science, high-pressure physics and mineralogy, solid-state chemistry and more, powering entire computational subdisciplines. Modern DFT simulation codes can calculate a vast range of structural, chemical, optical, spectroscopic, elastic, vibrational and thermodynamic phenomena. The ability to predict structure–property relationships has revolutionized experimental fields, such as vibrational and solid-state NMR spectroscopy, where it is the primary method to analyse and interpret experimental spectra. In semiconductor physics, great progress has been made in the electronic structure of bulk and defect states despite the severe challenges presented by the description of excited states. Studies are no longer restricted to known crystallographic structures. DFT is increasingly used as an exploratory tool for materials discovery and computational experiments, culminating in ex nihilo crystal structure prediction, which addresses the long-standing difficult problem of how to predict crystal structure polymorphs from nothing but a specified chemical composition. We present an overview of the capabilities of solid-state DFT simulations in all of these topics, illustrated with recent examples using the CASTEP computer program.

The observed reactivity of MgO with water is in apparent conflict with theoretical calculations which show that molecular dissociation does not occur on a perfect (001) surface. We have performed ab-initio total energy calculations which show that a chemisorption reaction involving a reconstruction to form a (111) hydroxyl surface is strongly preferred with ∆E = −90.2 kJ mol −1 . We conclude that protonation stabilizes the otherwise unstable (111) Magnesium oxide has long provided a prototype for the study of surface structure and chemical reactions of oxides. Naturally occurring MgO, known by its mineral name of periclase, is not a common crustal mineral, but its simple structure makes it an excellent example for the investigation of mineral surface chemistry.Reactions at mineral surfaces are responsible for much of the chemical change which occurs in the Earth's crust. Weathering reactions control the erosion of rocks and the consequent evolution of surface topography thus providing an opposing mechanism to the more dramatic process of mountain building. Aqueous reactions in sedimentary basins are responsible for the diagenetic processes which transform unconsolidated sediments into rocks. In this work we study the nature of a simple mineral surface when exposed to an aqueous environment and the chemical interaction of water with that surface. This is both a prerequisite to studying the interaction with aqueous solutions and a tractable first step towards ligand-exchange reactions in more complex silicate minerals.We have performed experiments on single-crystals of MgO prepared with high-quality (001) faces which were reacted with acidic solutions. The experiments and results are reported in detail elsewhere 1 , the main feature being the development of an altered surface layer. Elastic Recoil Detection Analysis (ERDA)2 shows protonation to a depth of 900Å with a H/Mg ratio close to 2 giving a probable chemical composition of magnesium hydroxide. Indeed brucite (the mineralogical name for Mg(OH) 2 ) is the most common alteration product of periclase in the natural environment 4 and well-crystallized intergrowths of brucite on periclase have been reported 5 . The initial stage in the reaction is hydroxylation of the surface. MgO has the cubic rocksalt structure with (001) cleavage planes. This is the most stable surface and is the only one seen experimentally 6 . The simplest possibility for a hydroxylated surface is obtained by dissociating a water molecule and placing the OH group above each magnesium ion and the H above each oxygen of the (001) surface (see Fig. 1a) as postulated by Coluccia et al.7 . Some striking hydroxylation experiments were reported by Jones et al. who studied surface roughening on (001) faces of nanocrystalline MgO in a transmission electron microscope 8 . The remarkable affinity of MgO for water is demonstrated by their in situ observation of hydration-induced surface roughening over 10 minutes under vacuum with P H 2 O < 10 −5 Pa. The presence of surface hydroxyl groups on MgO powde...

We report the synthesis, structure, and properties of novel bulk rhenium nitrides, hexagonal Re2N, and Re3N. Both phases have very high bulk moduli of >400 GPa, similar to the most incompressible binary transition-metal (TM) carbides and nitrides found to date. However, in contrast to other incompressible TM carbides and nitrides, Re3N is better placed for potential technological applications, as it can be formed at relatively moderate pressures (13-16 GPa) and temperatures (1600-2400 K).