Effect of basis set on the band structure and FERMI surface of BEDT-TTF-based organic conductors

Summary form only given. The effects of the basis set quality on the band structures and Fermi surfaces of organic salts have been investigated. The results concerning the salts for which the stacking mode involves some step chains, as in the /spl alpha/ or /spl beta/ series, are quite sensitive to the quality of the basis set. The shapes of the contours of the Fermi surface close to an edge may be shown to be strongly dependent on the ratio of the two integrals which define the intrachain interactions. As a consequence, the 2D character of the Fermi surface is decreased by using a single-/spl zeta/ basis set, contrary to the general expectation. In particular, the observed difference between /spl alpha/-ET/sub 2/[MHg(SCN) /sub 4/ for M=K or M=NH/sub 4/, might thus be related to slight changes in the stacking interactions which may be sufficient to induce pronounced changes in the Fermi surface. The 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 electronic band structures of other structural types as the /spl kappa/- structures or the /spl tau/ ones.