  We propose a framework to construct the ground-state energy and density
matrix of an N-electron system by solving selfconsistently a set of
single-particle equations. The method can be viewed as a non-trivial extension
of the Kohn-Sham scheme (which is embedded as a special case). It is based on
separating the Green's function into a quasi-particle part and a background
part, and expressing only the background part as a functional of the density
matrix. The calculated single-particle energies and wave functions have a clear
physical interpretation as quasiparticle energies and orbitals.
