EFFECTIVE ELECTRON HAMILTONIAN FOR A QUASI-ONE-DIMENSIONAL SYSTEM - THE (TMTSF)(2)X AND THE (TMTTF)(2)X SYSTEMS

In this paper we have introduced a variational approach to investigate the ground state of a model which includes both the Holstein electron -phonon interaction and the extended Hubbard electron-electron interac tion. We have considered a variational state for the phonon subsystem which generalizes the previous used forms. This state allows to take i nto account the possibility of extended phonon mediated correlations. The effective electron Hamiltonian, which we have obtained, includes f irst and second neighbor electron-electron interaction terms. We have treated exactly, through a Lanczos method, this effective model in the one-dimensional case. We have applied our method to two Bechgaard sal ts and in these cases we have estimated the correlation parameters. We have shown that the introduction of electron-phonon interaction allow s an estimate of the on site U and nearest-neighbor V Coulomb repulsio n, which are in agreement with the experimental optical spectra of the above mentioned two compounds.