Determination of the electric charge distribution in an amorphous material. II. Evaluation of the Madelung potentials and computation of the electric charges

All the empirical methods used to estimate the charge distribution in a given chemical composite relay on the possibility to describe the system through a set of chemical and physical parameters. For systems with known structure, some authors were able to reproduce the charge distribution on atoms in very good agreement with those worked out using ab initio calculations: the knowledge of the composite structure allow the use of orbital electronegativities or the exact computation of the electric potentials as functions of the atoms electric charge. On the other hand there are in literature some examples in which empirical methods are applied also in the case of disordered materials. In this case the lack of any structural symmetry prevent the exact calculation of both the orbital electronegativities and of the Madelung potentials and the results may be quite different in relation to the method applied. This article describe novel algorithms to improve the evaluation of the Madelung potentials in amorphous systems. Further, the introduction of an iterative process in the Electronegativity Equalization Method enables a more reliable estimate of the charge distribution.