We present a genetic algorithm for the atomistic design and globaloptimisation of substitutionally disordered bulk materials and surfaces.Premature convergence which hamper conventional genetic algorithms due toproblems with synchronisation is avoided using a symmetry adapted crossover.The algorithm outperforms previously reported Monte Carlo and genetic algorithmsimulations for finding low energy minima of two simple alloy models withoutthe need for any redesign.

We present the structural properties of thin films of a calcium alummosilicate glass generated by classical molecular dynamics (MD). The films are generated by two methods: in the first, the films are created in the liquid state and quenched to 300 K; in the second, the films are generated at room temperature. Depending on the method, film thickness and surface roughness are different but the main structural characteristics of the films are similar. The atomic concentration appears to be inhomogeneous from the center of the films to the surface and new structural entities are present at the film surfaces. The surfaces are depleted in calcium atoms and are enriched in aluminum atoms. This atomic arrangement induces a de-polymerized area under the surface. At the surface, all structural properties are modified in comparison with those of the bulk: interatomic distances, angular distributions, ring size, coordinations. In order to confirm the surface properties, we relaxed a surface using ab initio molecular dynamics. Some modifications appear but they do not significantly change the results obtained by classical MD, validating the use of interatomic potentials for the study of such films. (C) 2007 Elsevier B.V. All rights reserved.

A fitting scheme is proposed to obtain effective potentials fromCar-Parrinello molecular dynamics (CPMD) simulations. It is used toparameterize a new pair potential for silica. MD simulations with this newpotential are done to determine structural and dynamic properties and tocompare these properties to those obtained from CPMD and a MD simulation usingthe so-called BKS potential. The new potential reproduces accurately the liquidstructure generated by the CPMD trajectories, the experimental activationenergies for the self-diffusion constants and the experimental density ofamorphous silica. Also lattice parameters and elastic constants of alpha-quartzare well-reproduced, showing the transferability of the new potential.

First-principles simulations based on the density functional theory are used in order to generate silica glasses containing 3.84wt% water molecules employing different quench protocols. Using the Kohn-Sham density of states we find localized states in the band gap that can be associated with doubly occupied Si-O dangling bond (DB) which are negatively charged and are compensated by positively charged threefold coordinated oxygens. The position of these states above the O 2p valence band depends on the local environment of the dangling bonds, in particular on the presence of other defects in their neighborhood, and on the hydrogen bond length. These native defects, which could exist in optical fibers for instance, are compatible with the optical absorption and photoluminescence bands observed in amorphous silica and their dependence in the OH content. If present, these pre-existing defects would play a significant role as precursors in the laser-induced defect formation process. Copyright (c) EPLA, 2008.