The molecular crystal approach for pi-conjugated polymers: from the Pariser-Parr-Pople Hamiltonian to the Holstein model for polaron states

Starting from the pi-electron Pariser-Parr-Pople (PPP) Hamiltonian which in cludes both strong electron-phonon and electron-electron interactions, we p ropose some strongly correlated wave functions of increasing quality for th e ground states of conjugated polymers. These wave functions are built by c ombining different finite sets of local configurations extended over at mos t two nearest-neighbour monomers. With this picture. the doped case with on e additional particle is expressed in terms of quasi-particles. Thus, the p olaron formation problem reverts to the study of a Holstein-like model.