On the ab initio evaluation of Hubbard parameters. II. The κ-(BEDT-TTF)2Cu[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 $\mathrm{\kappa -(}{\mathrm{BEDT-TTF)}}_{\mathrm{2}}{\mathrm{Cu[N(CN)}}_{\mathrm{2}}\mathrm{]Br}$ 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 $\mathrm{(X}$ and $\mathrm{W)}$ 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 Hückel Theory (whereas the minimal basis set estimates are completely unreliable). On the other hand, the $U_a^{\mathrm{eff}}$ values exhibit an appreciable dependence on the chemical environment.