ELECTRONIC AND PHONONIC STATES OF THE HOLSTEIN-HUBBARD DIMER OF VARIABLE-LENGTH

We consider a model Hamiltonian for a dimer of length a including all the electronic one- and two-body terms consistent with a single orbita l per site, a free Einstein phonon term for a frequency Omega, and an electron-phonon coupling go of the Holstein type. The bare electronic interaction parameters were evaluated in terms of Wannier functions bu ilt from Gaussian atomic orbitals. An effective polaronic Hamiltonian was obtained by an unrestricted displaced-oscillator transformation, f ollowed by evaluation of the phononic terms over a squeezed-phonon var iational wave function. For the cases of quarter-filled and half-fille d orbitals, and over a range of dimer length values. the ground state for given g(0) and Omega was identified by simultaneously and independ ently optimizing the orbital shape, the phonon displacement, and the s queezing effect strength. As a varies, we generally find discontinuous changes of both electronic and phononic states, accompanied by an app reciable renormalization of the effective electronic interactions acro ss the transitions, due to the equilibrium shape of the wave functions strongly depending on the phononic regime and on the type of ground s tale. [S0163-1829(98)06035-4].