EXTENDED HUBBARD-MODEL AT STRONG-COUPLING

The half-filled extended Hubbard model (containing nearest-neighbor in teraction) is considered at strong coupling in fourth-order perturbati on theory. The ground-state energies in the spin- and charge-density-w ave phases and the resulting phase diagram are calculated in various d imensions: here d = 1, d = infinity, high dimensions (d much greater t han 1), and d = 2,3 are discussed separately. In d = 1 it is shown tha t, for U greater than or similar to 6, fourth-order perturbation theor y leads to excellent agreement with existing Monte Carlo data. Second- order perturbation theory alone is valid only at unrealistically large values of U and V. In d = infinity, one needs fourth-order perturbati on theory to obtain any nontrivial contribution to the phase diagram a t all. As a consequence the fourth-order corrections to the ground-sta te energy and to the phase diagram are large also in d = 2,3. The form alism used to obtain the perturbative expansion is discussed in an app endix.