Random-phase approximation for multi-band Hubbard models

We derive the random-phase approximation for spin excitations in general mu lti-band Hubbard models, starting from a collinear ferromagnetic Hartree-Fo ck ground state. The results are compared with those from a recently introd uced variational many-body approach to spin waves in itinerant ferromagnets . As we exemplify for Hubbard models with one and two bands, the two approa ches lead to qualitatively different results. The discrepancies can be trac ed back to the fact that the Hartree-Fock theory fails to describe properly the local moments which naturally arise in a correlated-electron theory.