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We employ density-functional theory to calculate realistic parameters for an extended Hubbard model of the molecular metal TTF-TCNQ. Considering both intra- and intermolecular screening in the crystal, we find significant longer-range Coulomb interactions along the molecular stacks, as well as inter-stack coupling. We show that the long-range Coulomb term of the extended Hubbard model leads to a broadening of the spectral density, likely resolving the problems with the interpretation of photoemission experiments using a simple Hubbard model only., Comment: 4 pages, 2 figures

The proposal of using the field-effect for doping organic crystals has raised enormous interest. To assess the feasibility of such an approach, we investigate the effect of a strong electric field on the electronic structure of C60 crystals. Calculating the polarization of the molecules and the splittings of the molecular levels as a function of the external field, we determine up to what field-strengths the electronic structure of C60 stays essentially unchanged, so that one can speak of field-effect doping, in the sense of putting charge carriers into otherwise unchanged states. Beyond these field strengths, the electronic structure changes so much, that on can no longer speak of a doped system. In addition, we address the question of a metal-insulator transition at integer dopings and briefly review proposed mechanisms for explaining an increase of the superconducting transition temperature in field-doped C60 that is intercalated with haloform molecules., Comment: 5 pages, 3 figures; proceedings of the IWEPNM'03, Kirchberg

Fullerides are crystals made from C60 molecules. When doped with alkali metals the valence electrons of the alkali atoms are transfered to the C60 molecules. We show how to set up a model Hamiltonian to describe the correlated hopping of these electrons between C60 molecules. Then we discuss Monte Carlo methods for such lattice Hamiltonians, putting special emphasis on the optimization of Gutzwiller wavefunctions both in variational and fixed-node diffusion Monte Carlo. As application of these methods we analyze the metal-insulator transition in the integer-doped Fullerides., Comment: 16 pages, 10 eps figures included; Proceedings of the NATO ASI Quantum Monte Carlo Methods in Physics and Chemistry, Cornell University, July, 12-25, 1998; additional information is available at http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene/