ELECTRONIC AND PHONONIC TRANSITIONS IN THE 2-SITE HOLSTEIN MODEL

We consider the Hamiltonian for a dimer including all the electronic o ne-and two-body terms for a single orbital, the free phonon term for a single frequency Omega, and an electron-phonon coupling g of the Hols tein type. The bare electronic interaction parameters were evaluated i n tel ms of Wannier functions, built from atomic Gaussian orbitals. An effective polaronic Hamiltonian was obtained by a diaplaced-oscillato r transformation, followed by evaluation of the phononic terms over a squeezed-phonon variational wavefunction. Its eigenvalues and eigenvec tors have been obtained in explicit form for N = 1, 2, 3, 4 electrons. For each N and over a range of dimer length values, the ground state for given g and Omega was identified by optimizing the orbital shape a nd the squeezing effect strength. We find, for any N if g not equal 0, an abrupt transition from a delocalized to a localized polaron phase, due to a discontinuous collapse of the squeezing effect above a criti cal dimer length. Also, in the case N = 2, and for a different dimer l ength, we find a second transition from a singlet to a triplet ground state that depends also on g. All the effective electronic interaction s are appreciably renormalized across the transitions, as the equilibr ium shape of the wavefunctions strongly depends on both the squeezing effect and the type of ground state.