<i>Ab initio</i> Derivation of Low-Energy Model for κ-ET Type Organic Conductors

Abstract

We derive effective Hubbard-type Hamiltonians of $\\kappa$-(ET)$_2X$, using an\n{\\em ab initio} downfolding technique, for the first time for organic\nconductors. They contain dispersions of the highest occupied Wannier-type\nmolecular orbitals with the nearest neighbor transfer $t$$\\sim$0.067 eV for a\nmetal $X$=Cu(NCS)$_2$ and 0.055 eV for a Mott insulator $X$=Cu$_2$(CN)$_3$, as\nwell as screened Coulomb interactions. It shows unexpected differences from the\nconventional extended H\\"uckel results, especially much stronger onsite\ninteraction $U$$\\sim$0.8 eV ($U/t$$\\sim$12-15) than the H\\"uckel estimates\n($U/t$$\\sim$7-8) as well as an appreciable longer-ranged interaction.\nReexamination on physics of this family of materials is required from this\nrealistic basis.\n