Coulomb effects in disordered solids. II. a simplified computational method

We show that Coulomb sums can be truncated in real space in disordered ionic solids, when calculating response functions such as the density of states() and the dielectric function (k,). The Coulomb forces are treated exactly inside a sphere of radius Rc around each ion. Outside this sphere, the solid is treated as a continuously polarizable medium, which leads to the usual shape-dependent effects. This method is shown to be computationally and conceptually simpler than a previous method used by us on a large periodic random network representing silica. The method is also illustrated on a diluted lattice of polarizable point dipoles. The method works for all quantities, although special care must be taken with (k,) for small k, because of the long-ranged coherence of the eigenstates involved.