As a side note to the question of calculating atomic charges, there
has been a lot of discussion in the literature of methods for structural
prediction for van der Waals dimers. It's generally accepted that
electrostatic interactions are a major contributor to the overall
interaction. In general, methods based on atomic charges alone do not
predict the structures or binding energies well. Furthermore, methods
based on the distributed multipole model of Buckingham, Fowler and Stone
appear to work fairly well only when one of the partners of the dimer is
non-polar (e.g. in C2H4 - SO2), but usually fails when both components
are polar (as in (SO2)2).
What does it all mean? My best interpretation is that you need to be
very careful when applying simple electrostatic models to intermolecular
interactions. (Profound, isn't it?) So, I emphasize the cautions given
about charge calculations - reliability is hard to define and harder to
quantify (and, unfortunately, often impossible to achieve).