title: Toward the parametrization of the Hubbard model for salts of bis(ethylenedithio)tetrathiafulvalene: a density functional study of isolated molecules.
abstract: We calculate the effective Coulomb repulsion between electrons/holes U(m) ((v)) and site energy for an isolated bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecule in vacuo. U(m) ((v))=4.2+/-0.1 eV for 44 experimental geometries taken from a broad range of conformations, polymorphs, anions, temperatures, and pressures (the quoted "error" is one standard deviation). Hence we conclude that U(m) ((v)) is essentially the same for all of the compounds studied. This shows that the strong (hydrostatic and chemical) pressure dependence observed in the phase diagrams of the BEDT-TTF salts is not due to U(m) ((v)). Therefore, if the Hubbard model is sufficient to describe the phase diagram of the BEDT-TTF salts, there must be significant pressure dependence on the intramolecular terms in the Hamiltonian and/or the reduction in the Hubbard U due to the interaction of the molecule with the polarizable crystal environment. The renormalized value of U(m) ((v)) is significantly smaller than the bare value of the Coulomb integral, F(0)=5.2+/-0.1 eV, across the same set of geometries, emphasizing the importance of using the renormalized value of U(m) ((v)). The site energy (for holes), xi(m)=5.0+/-0.2 eV, varies only a little more than U(m) ((v)) across the same set of geometries. However, we argue that this variation in the site energy plays a key role in understanding the role of disorder in bis(ethylenedithio)tetrathiafulvalene salts. We explain the differences between the beta(L) and beta(H) phases of (BEDT-TTF)(2)I(3) on the basis of calculations of the effects of disorder.
authors.affiliation: Centre for Organic and Photonic Electronics, School of Physical Sciences, University of Queensland, Queensland 4072, Australia. edan@physics.uq.edu.au
authors.forename: Edan
authors.initials: E
authors.lastname: Scriven
authors.forename: B J
authors.initials: BJ
authors.lastname: Powell