DETERMINATION OF THE COULOMBIC INTERACTION PARAMETERS OF THE EXTENDEDHUBBARD-MODEL IN THE ORGANIC CONDUCTORS

A mixed Valence-Bond/Hartree-Fock method applied to all valence electr ons calculations on finite size clusters was derived in order to size up the relative magnitude of the coulombic interactions between conduc tion electrons in organic conductors. An effective Hamiltonian built u pon a single site orbital per molecule reproduces the whole set of VB energy levels. This extraction is most consistent when molecular orbit als optimized on monomers are used. Coulombic parameters describing th e interactions in clusters of organic molecules derived of TTF (tetrat hiafulvalene) may be thus extracted. The magnitude of the coulombic te rms (U approximate to 4 eV; V (nearest-neighbour) = 2 - 3 eV) differs qualitatively from previous estimations and evidences the long range p art of the potential. The calculated charge transfer excitations are s till in good agreement with the experimental results.