A Green function method for the intermediate valence model

Abstract
An approximate contraction is defined which enables the perturbation series for the intermediate valence model to be summed to all orders. Two forms of Coulomb correlation, U, are considered and in both cases it is demonstrated that the Fermi level at absolute zero cannot lie in the gap of the pseudo-particle energy spectrum. A general expression is derived for the ground state energy. Finally, the validity of the method is examined when the number of lattice sites, N, tends to infinity.

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