  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.
