  We present an efficient method for incorporating the dynamical effects of the
screening of the Hubbard U by electronic degrees of freedom in the solid into
the single site dynamical mean field approximation. The formalism is
illustrated by model system calculations which capture the essential features
of the frequency dependent interactions proposed for Gd, Ni, SrVO_3 and other
compounds. Screening leads to shifts in the metal-insulator phase boundary,
changes in the spectral function near the Mott-Hubbard gap edge and to a
renormalization of the quasiparticle weight. Hubbard bands are generically
neither separated by the screened nor the unscreened interaction energy,
implying that the common practice of extracting the Hubbard U from the energies
of features in photoemission and inverse photoemission spectra requires
reexamination.
