  We consider a model Hamiltonian for a dimer including all the electronic one-
and two-body terms consistent with a single orbital per site, a free Einstein
phonon term, and an electron-phonon coupling of the Holstein type. The bare
electronic interaction parameters were evaluated in terms of Wannier functions
built from Gaussian atomic orbitals. An effective polaronic Hamiltonian was
obtained by an unrestricted displaced-oscillator transformation, followed by
evaluation of the phononic terms over a squeezed-phonon variational wave
function. For the cases of quarter-filled and half-filled orbital, and over a
range of dimer length values, the ground state was identified by simultaneously
and independently optimizing the orbital shape, the phonon displacement and the
squeezing effect strength. As the dimer length varies, we generally find
discontinuous changes of both electronic and phononic states, accompanied by an
appreciable renormalization of the effective electronic interactions across the
transitions, due to the equilibrium shape of the wave functions strongly
depending on the phononic regime and on the type of ground state.
