(533e) Atomistic Charge-Transfer Potentials for Silica and Aqueous Silicates

The development and performance of charge-transfer potentials for vitreous silica, crystalline silica polymorphs, hydrated silica sol-gel, and water is considered. Charge-transfer potentials allow for redistribution of charge between atoms and therefore can describe their behavior with changes in local chemical environment; such behavior is critical to correct description of, for instance, heterolytic bond breaking. Alavi, Alvarez, Elliott and McDonald [Phil. Mag. B., 65, 489 (1992) previously developed a charge-transfer model for vitreous silica. We have significantly modified this model in order in order to promote silicon-oxygen bond stability in the isobaric-isothermal ensemble and to more accurately reproduce crystal lattice constants. The analytic calculation of forces and pressure in this potential is non-trivial and discussed in detail.

Parameterization of a complete potential using both charge-transfer functions and traditional two- and three-body empirical potential terms is a challenging numerical problem. Evolutionary Strategy (ES) optimizations were used to fit the charge-transfer potential against first-principles data. Evolutionary strategies are similar to genetic algorithms in that they use parameter recombination and mutation in many simultaneous trial solutions with a competitive, generation-based search. This new model is then tested in multiple systems and state points, focusing on hydrous and anhydrous vitreous silica, and crystalline silica. Properties such as melting points, radial and angular distribution functions, and average coordination are calculated. Initial steps of silicic acid gelation are also simulated. The results are compared to the Feuston-Garofalini [J. Phys. Chem., 94, 5351 (1990)] and the van Beest, Kramer and van Santen (BKS) [Phys. Rev. Lett., 64, 1955 (1990)] models for silica systems.