THE GUTZWILLER APPROXIMATION FOR DEGENERATE BANDS - A FORMAL DERIVATION

We present a detailed derivation of the Gutzwiller approximation for m ulti-band Hubbard models with density-density Coulomb interactions. Fo r the one-band Hubbard model we introduce a mathematically well-define d formalism which is easily generalized to the multi-band case. In con trast to earlier attempts, our approach allows us to include inter-orb ital hopping terms in the Hamiltonian.