Effective Coulomb interactions within BEDT-TTF dimers

We calculate the parameters for Hubbard models of κ-(BEDT-TTF)₂X and β-(BEDT-TTF)₂X. We use density-functional theory (DFT) to calculate the interactions between holes in dimers of the organic molecule bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) for 23 experimental geometries taken from a range of materials in both the β and κ polymorphs. We find that the effective Coulomb interactions are essentially the same for all of the compounds studied. We highlight the disagreement between our parametrization of the Hubbard model and previous results from both DFT and Hückel methods. We show that this is caused by the failure of an assumption made in previous calculations (which estimate the effective Coulomb interaction from the intradimer hopping integral). We discuss the implications of our calculations for theories of the BEDT-TTF salts based on the Hubbard model and use our calculated parameters to explain a number of phenomena caused by conformational disorder in these materials.