Effect of the intersite Coulomb interaction in the Hubbard-Holstein model on a four-site chain

We derive an effective polaronic Hamiltonian by applying the displacement a nd the squeezing transformations to an extended version of the Hubbard-Hols tein model, which includes all the electronic one- and two-body interaction s for electrons coupled to dispersionless optical phonons. This effective H amiltonian is then exactly solved on a four-site chain and the effect of th e intersite Coulomb interaction on the spin and charge ordering in the grou nd state at half-filling is investigated. (C) 2000 Elsevier Science B.V. Al l rights reserved.