Nonlocal effects in the metal-insulator transition beyond the Hubbard III approximation

Combining the positive features of the spectral density approach and the Hubbard III approximation we propose a self-energy ansatz by which nonlocal effects are introduced beyond the Hubbard III approximation. The solution with the self-energy is shown to preserve the first four moments, as in the spectral density approach with a two-pole ansatz. The influence of the nonlocal effects introduced on the spectral functions and the metal-insulator transition are discussed. It is found that the nonlocal effects lead to significant difference of the density of states from the Hubbard III approximation near the Fermi level at intermediate interaction, which makes the critical interaction for the metal-insulator transition to be somewhat greater than the bandwidth.