New approach to the theory of intermediate valence. I. General formulation

We present a new approach to the theory of intermediate valence, applicable to a specific kind of such solids. It consist of: (i) diagonalization of all intra-atomic terms in the Hamiltonian, including hybridization; (ii) elimination by means of a projection of all states beyond a low-energy manifold; (iii) conversion of the remaining states into equivalent fermion states; (iv) expression of the intra- and interatomic states in terms of the new fermion operators; and (v) treatment of this new Hamiltonian in a mean-field approximation. This approach, although valid in restricted cases, avoids all the problems of the alternative treatments.