EFFECT OF BASIS-SET ON THE BAND-STRUCTURE AND FERMI-SURFACE OF BEDT-TTF-BASED ORGANIC CONDUCTORS

The calculation of the Fermi surfaces of organic salts by different basis sets has been used as a theoretical tool to evidence peculiar structural characteristics which might influence deeply the electronic properties of these systems. The alpha-ET2[MHg(SCN)4] salts are quite unique because their stacking mode induces large transfer integrals or almost zero ones for the intrachain interactions. This results in a marked difference between the single-zeta and the double-zeta Fermi surfaces which has been explained by means of simple geometrical factors. The implication of this ''structural sensitivity'' has been discussed in relation with the low temperature ground states in the alpha-series. The observed difference between the K and the NH4 salts might thus be related to slight changes in the stacking interactions which may be sufficient to induce pronounced changes in the Fermi surface. A proper analysis of the electronic properties of these salts thus requires the determination of the low T crystal structures. Finally, the basis set effect has been shown to have little influence on the band structures Of kappa-type salts.