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Effective Coulomb interactions within BEDT-TTF dimers

We calculate the effective Coulomb interactions between holes in dimers of the organic molecule BEDT-TTF in vacuo. We use density functional theory (DFT) to parameterise Hubbard models for beta and kappa phase organic charge transfer salts. We focus on the intra-dimer Coulomb repulsion, U_d, and the inter-monomer Coulomb repulsion, V_m. We find that U_d = 3.22 \pm 0.09 eV and V_m = 2.71 \pm 0.10 eV for 23 experimental geometries taken from a range of materials in both polymorphs. The quoted error is one standard deviation over the set of conformations studied. We conclude that U_d and V_m are essentially the same for an isolated dimer with the geometries present in all of the compounds studied. We highlight the disagreement between our parameterisation of the Hubbard model and previous results from both DFT and Huckel methods and show that this is caused by the failure of an assumption made in previous calculations (that U_m >> V_m, where U_m is the effective intra-monomer Coulomb repulsion). We discuss the implications of our calculations for theories of the BEDT-TTF salts based on the Hubbard model on the 2D anisotropic triangular lattice and explore the role of conformational disorder in these materials.