NUMERICAL STUDY OF SYSTEMS WITH DIAGONAL AND OFF-DIAGONAL DISORDER IN A NON-ORTHOGONAL BASIS

Simple structurally disordered materials were studied, taking into account diagonal and off-diagonal disorder in a non-orthogonal basis. The disordered systems were represented by random two-dimensional distributions of identical atoms. Calculations were performed by the iterative extended Huckel method, which allows for the inclusion of intra-atomic electronic repulsion. Densities of states, localisation of the wavefunctions and redistribution of charges due to the disorder were evaluated as a function of the density (or the disorder, as these two variables are taken to be inversely proportional). It was found that a metal-non-metal transition due to the disorder also occurred in these calculations with a non-orthogonal basis, and for a value of the density in agreement with experiments. Although the system studied was one of identical atoms, the effect of the disorder was to produce a different field around each atom and consequently some atoms become positively charged and others negatively charged. The distribution of charges obtained was asymmetric, with most atoms having charges between plus and minus 0.2e, and it became wider as the density decreased.