PAIR BINDING SMALL CLUSTERS DESCRIBED BY THE EXTENDED HUBBARD-MODEL

We calculate the binding of two added electrons or holes for several c lusters described by the extended Hubbard model. Pairing is found only away from half filling and for the geometries which give particle-hol e symmetric spectra. In these situations the intersite repulsion is es sential and pairing depends on it very strongly. We conclude that smal l-cluster results are adequate to answer the question of whether the b inding is possible only if the one-particle spectra are qualitatively similar to those of the considered lattice. Altogether, the results su ggest that a purely electronic mechanism of superconductivity is not l ikely in alkali metal doped fullerides.