  Multi-band Gutzwiller-correlated wave functions reconcile the contrasting
concepts of itinerant band electrons versus electrons localized in partially
filled atomic shells. The approximate evaluation of these variational ground
states becomes exact in the limit of large coordination number. The result
allows the identification of quasi-particle band structures for correlated
electron systems. As a first application, we summarize a study of itinerant
ferromagnetism in a two-band model, thereby elucidating the co-operation of the
Coulomb repulsion and the Hund's-rule exchange. Then, we present results of
calculations for ferromagnetic nickel, using a realistic 18 spin-orbital basis
of 4s, 4p and 3d valence electrons. Good agreement with the experimental
ground-state properties of nickel is obtained. In particular, the
quasi-particle energy bands agree much better with the photo-emission and Fermi
surface data than the band structure obtained from spin-density functional
theory. Finally, we present results for the variational spinwave dispersion for
our two-band model.
