Hubbard Model. I. Degeneracy in the Atomic Light

The atomic limit of the Hubbard Hamiltonian is extremely degenerate. The ground-state degeneracy, in particular, makes the zero-temperature Green's function for this limit ambiguous. The manifestations of the degeneracy in the one- and two-particle Green's functions are found. The degeneracy gives rise to nonlocal spatial correlation of physical significance which must be taken into account in narrow-band-regime perturbation schemes for the model.