THE IONIC MODEL - EXTENSION TO SPATIAL CHARGE-DISTRIBUTIONS

In this paper the validity of the classical ionic model, using a Madel ung term and a Born-Mayer repulsive term, is investigated quantatively for systems with a considerable overlap of the electron clouds of nei ghbouring ions, such as silicates with a high degree of polymerisation . A modified ionic model is presented which takes into account the spa tial extent of the ions within the approximation of spherical atoms. B oth models are tested against quantum mechanical electron densities an d energies for SiO44--clusters. The data demonstrate the validity of t he Spherical atom approximation, producing a fit of 99.995%, and the i mportance of many-body effects maintaining the spherical symmetry of t he electron clouds as contraction/expansion of the ions and charge tra nsfer between ions. Although the new interaction potential is physical ly more plausible than the classical Born-Mayer model, both models rep roduce the quantum mechanical potential surface with numerical accurac ies of the same order of magnitude. The new model provides an improved tool for judging between ionic and non-ionic effects and for analysis of the quantum mechanical electron densities and interaction energies .