  We present a general method for determining screened Coulomb parameters in
molecular assemblies, in particular organic molecular crystals. This allows us
to calculate the interaction parameters used in a generalized Hubbard model
description of correlated organic materials. In such a model only the electrons
in levels close to the Fermi level are included explicitly, while the effect of
all other electrons is included as a renormalization of the model parameters.
For the Coulomb integrals this renormalization is mainly due to screening. For
molecular materials we can split the screening into intra- and inter-molecular
screening. Here we demonstrate how the inter-molecular screening can be
calculated by modeling the molecules by distributed point-polarizabilities and
solving the resulting self-consistent electrostatic screening problem in real
space. For the example of the quasi one-dimensional molecular metal TTF-TCNQ we
demonstrate that the method gives remarkably accurate results.
