  The strong-coupling perturbation theory (SCPT) for correlated electron
systems is extended to the case of full Coulomb interaction. The Coulomb
mechanism of the orbital polarization is discussed and attention is paid to the
importance of spectral weight transfer between the localized and delocalized
subsystems of electrons. A one-to-one correspondence between subsets of Feynman
graphs of SCPT (which we name the approximation of renormalized Fermions (ARF))
and weak-coupling perturbation theory (WCPT) is established. The comparison of
the Galitskii and Migdal expression for the total energy and the Sham equation
(which connects the self-energy and the exchange-correlation potential in
density functional theory) for WCPT with the ones for the systems with strong
electron correlations is used for the formulation of a simple theory for
extending the local density approximation to density functional theory (DFT) to
include explicitly correlations. The extension requires the inclusion of the
many-electron spectral weights in the definition of the charge density and the
renormalization of the mixing and hopping matrix elements caused by many body
effects.
