抄録
We derive effective Hubbard-type Hamiltonians of κ-(BEDT-TTF)2X, using an ab initio downfolding technique, for the first time for organic conductors. They contain dispersions of the highest occupied Wannier-type molecular orbitals with the nearest neighbor transfer t∼0.067 eV for a metal X=Cu(NCS)2 and 0.055 eV for a Mott insulator X=Cu2(CN)3, as well as screened Coulomb interactions. It shows unexpected differences from the conventional extended Hückel results, especially much stronger onsite interaction U∼0.8 eV (U⁄t∼12–15) than the Hückel estimates (U⁄t∼7–8) as well as an appreciable longer-ranged interaction. Reexamination on physics of this family of materials is required from this realistic basis.
