  We generalize the screened Korringa-Kohn-Rostoker (SKKR) method for solving
the corresponding Kohn-Sham-Bogoliubov-de Gennes (KSBdG) equations for surfaces
and interfaces. As an application of the newly developed theory we study the
quasiparticle spectrum of Au overlayers on a Nb(100) host. We find that, within
the superconducting gap region, the quasiparticle spectrum consists of Andreev
bound states (ABS) with a dispersion which is closely connected to the
underlying electronic structure of the overlayer. We also find that the
spectrum has a strongly k-dependent induced gap. The properties of the gap is
discussed in relation to the thickness of the overlayer, and it is shown that
certain states do not participate in the Andreev scattering process.
