On the ab initio evaluation of Hubbard parameters .2. The kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br crystal

A previously proposed approach for the ab initio evaluation of Hubbard parameters is applied to BEDT-TTF dimers. The dimers are positioned according to four geometries taken as the first neighbors from the experimental data on the kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br crystal. RHF-SCF, CAS-SCF and frozen-orbital calculations using the 6-31G(**) basis set are performed with different values of the total charge, allowing us to derive all the relevant parameters. It is found that the electronic structure of the BEDT-TTF planes is adequately described by the standard Extended Hubbard Model, with the off-diagonal electron-electron interaction terms (X and W) of negligible size. The derived parameters are in good agreement with available experimental data. Comparison with previous theoretical estimates shows that the t values compare well with those obtained from Extended Huckel Theory (whereas the minimal basis set estimates are completely unreliable). On the other band, the U-a(eff) values exhibit an appreciable dependence on the chemical environment. (C) 1997 American Institute of Physics.