  The prediction of material-specific properties of superconducting systems
such as the electronic structure and the transition temperature is one of the
major challenge in modern solid-state physics. In this paper we present the
first material specific calculations of realistic superconductor -- normal
metal heterostructures using density functional theory. In particular, we
calculate the quasiparticle spectrum of different normal metal overlayers on a
Nb(100) host. We find that the Andreev reflection leads to the formation of
momentum dependent quasiparticle bands in the normal metal. As a consequence,
the spectrum has a strongly momentum dependent induced gap. From the thickness
dependence of the gap size we calculate the superconducting critical
temperature of Au/Nb(100) heterostructures where we find very good agreement
with experiments. Moreover, predictions are made for similar heterostructures
of other compounds.
