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 ba sis sets has been used as a theoretical tool to evidence peculiar stru ctural characteristics which might influence deeply the electronic pro perties of these systems. The alpha-ET2[MHg(SCN)4] salts are quite uni que because their stacking mode induces large transfer integrals or al most zero ones for the intrachain interactions. This results in a mark ed difference between the single-zeta and the double-zeta Fermi surfac es which has been explained by means of simple geometrical factors. Th e 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 su fficient to induce pronounced changes in the Fermi surface. A proper a nalysis 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.