Inapplicability of the Hubbard Model for the description of real strongly correlated electrons

Physica B 199&200, 366 (1994).

The single band Hubbard model assumes that two electrons in an atom will doubly-occupy the same atomic orbital that is singly occupied when a single electron is at the atom. This assumption is incorrect, due to the fact that the spacing of atomic energy levels is always smaller than the intra-atomic Coulomb repulsion between electrons. Relaxation of this assumption leads to an effective single-band Hamiltonian where the interatomic hopping amplitude varies with the atomic charge occupation. First principles calculations of the variation in the magnitude of hopping amplitudes are presented and its possible consequences for superconductivity are discussed.