THE IONIC MODEL - EXTENSION TO SPATIAL CHARGE-DISTRIBUTIONS, DERIVATION OF AN INTERACTION POTENTIAL FOR SILICA POLYMORPHS

An interatomic interaction potential for silica polymorphs is derived based on the SCD model (cfr. Tijskens et al. 1994). This interaction p otential incorporates all classical electrostatic interactions arising from the spherical part of the spatial extent of the atoms including many body interactions. The potential is derived from Hartree-Fock ene rgies and electron densities for a set 72 [SiO4](4-)- and [Si2O7](6-) -clusters with variable configuration. The long range impact of the su rroundings on these clusters in the infinite system has been successfu lly mimicked by embedding the clusters in a finite three-dimensional a rray of point charges. This three-dimensional array of point charges i s optimized as to reproduce the average site potential and its gradien t occurring in II-IV-coordinated silica polymorphs at the central atom s of the clusters. The resulting interaction potential consists of two functions of the configurational coordinates, R, describing spherical ''atomic'' electron densities, sigma(A)(x, R) for A = Si, O. All clas sical electrostatic interactions are derived from these densities. A B orn-Mayer type correction term Delta E(qm)(R) models the quantum mecha nical interactions and the electrostatic interactions arising from the non-spherosymmetrical component of the electron density. The new inte raction potential model shows a slightly improved reproduction of the potential surface with respect to the classical Born-Mayer ionic model and demonstrates the importance of many body interactions as charge t ransfer and expansion/contraction of the atomic electron densities in these systems. Also the dependence of the quantum mechanical correctio n term Delta E(qm)(R) on the Si-O-Si-bond angle proves covalent effect s to be larger than suggested by the classical Born-Mayer ionic model thereby clarifying the controversy in literature on the importance of covalent effects in silica polymorphs and polymerised silicates in gen eral.