Abstract
The single-site approximation scheme of Blackman, Esterling, and Berk (BEB) is extended to the calculation of two-particle Green's functions using Baym's functional derivative technique. In BEB's representation, the irreducible four-point vertex is found to be site diagonal, and to have the correct form to guarantee that all conservation laws are satisfied. The electrical conductivity is derived using a Kubo-Greenwood formula, and is especially investigated for the case where hopping between atoms of different type is not allowed. In this case, the conductivity may be calculated analytically, and no mobility edge is found inside the band for any concentration.