A semiempirical simulation methodology: atomistic and ionic dissociation of silica

Modeling defects, surfaces and interfaces of silica is complicated by the p resence of charged and uncharged states of the atoms, and the existence of local nonstoichiometric structure. The changes in the local molecular envir onments associated with defects and surfaces are a consequence of changes i n the electronic structure away from the bulk values. The ability to captur e the electronic structure changes in simulations is highly desirable to st udy defects and surfaces. Moreover, the properties and problems of interest also require the use of large scale simulations. Hence, a semiempirical me thodology is introduced that contains these desirable properties. The model approach is described and results using the model in the absence of long r ange interactions are presented. The motivation to ignore Coulombic interac tions is driven by the desire to understand the model's ability to describe the bonding interactions and to what degree it captures any of the medium and long range order. (C) 1999 Elsevier Science B.V. All rights reserved.