  Dynamical mean field theory combined with finite-temperature exact
diagonalization is shown to be a suitable method to study local Coulomb
correlations in realistic multi-band materials. By making use of the sparseness
of the impurity Hamiltonian, exact eigenstates can be evaluated for
significantly larger clusters than in schemes based on full diagonalization.
Since finite-size effects are greatly reduced this approach allows the study of
three-band systems down to very low temperatures, for strong local Coulomb
interactions and full Hund exchange. It is also shown that exact
diagonalization yields smooth subband quasi-particle spectra and self-energies
at real frequencies. As a first application the correlation induced charge
transfer between t2g bands in Na_0.3CoO_2 is investigated. For both Hund and
Ising exchange the small eg' Fermi surface hole pockets are found to be
slightly enlarged compared to the non-interacting limit, in agreement with
previous Quantum Monte Carlo dynamical mean field calculations for Ising
exchange, but in conflict with photoemission data.
