2-BAND MODEL FOR HALOGEN-BRIDGED MIXED-VALENCE TRANSITION-METAL COMPLEXES .2. ELECTRON-ELECTRON CORRELATIONS AND QUANTUM PHONONS

We study the effects of electron-electron interactions in halogen-brid ged mixed-valence transition-metal linear-chain complexes (MX chains) applying a simple 3/4-filled two-band discrete tight-binding (extended ) Peierls-Hubbard model with both on-site and intersite electron-phono n couplings. We employ a variety of methods: perturbation theory, Hart ree-Fock approximation, and exact diagonalization in the limit of clas sical adiabatic phonons, and a variational approach allowing for a fin ite phonon frequency, i.e., quantum phonons and isotope effect. This v ariety of methods has proven necessary to obtain a complete picture, d ue to the structural richness of this model. We investigate the compet ition between the electron-electron and electron-phonon interactions i n a wide range of parameter regimes for both ground and excited states . We focus on values relevant to the MX chains, probing the experiment al variation as X and M are varied among X = Cl, Br, and I and M = Pt, Pd, and Ni (spanning electron-phonon-interaction-dominated to electro n-electron-interaction-dominated materials).