Toward the parametrization of the Hubbard model for salts of bis(ethylenedithio)tetrathiafulvalene: A density functional study of isolated molecules

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.