A Green-function diagrammatic method is utilized to calculate the occupation number of a narrow (j) band in a mixed-valence system. The method is a generalization of the Hubbard-1 approximation for a two-band Hamiltonian. The results at T=O K indicate the existence of an intermediate-valence state for finite hybridization between the bands, and abrupt transitions for values of G comparable to the half bandwidth of the conduction band. A metal-insulator transition is also obtained.
Physical review. B, Condensed matter. New York. Vol. 29, no. 6 (Mar. 1984), p. 3085-3089