Re: CCL:G:Charges

 > postulates.) are most appropriate.  The fitting of the electrostatic
 > potential of a
 > molecule is a highly non-trivial subject ( see e.g. Michelle Miller Francl
 > et al.,
 > J. Comput. Chem., Vol. 17, 1996, pp. 367-383, and the very recent review by
 > Sarah L. Price, J. Chem. Soc., Faraday Trans., Vol. 92, 1996, pp. 2997-3008
 > )
 > and in my view not a really solved problem, especially for large molecules.
 I don't agree. The charge fitting problem is not essentially different
 from other fitting problems in all branches of science and
 engineering, and lots of articles and books have been written about
 it. There are well established and robust techniques to find optimal
 charges and to judge to what degree they are well determined by the
 reference data. Much of this is not generally known in the computational
 chemistry community, but that's another problem.
 There remains of course the problem that the electrostatic potential
 in a reasonable region may not be sufficient to determine all charges
 uniquely. But that is not really fatal: if after adding all reference
 data you can reasonably get (e.g. from multiple conigurations) some
 charges still remain illdetermined, then they probably don't matter
 much in real life. If they do, then probably the fixed point charge model
 is not good enough.
 > are quite unknown to the community. Hall et al. fit the calculated electron
 > densities
 > of molecules to atom-centered gaussians as it is done by many DFT-programs
 > ( "density fitting with an auxillary basis" ) and got a very good
 ( in my
 > view an
 > excellent ) representation of the electrostatic potential of a molecule.
 Essentially that means choosing a more complicated model (gaussian densities
 rather than point charges), and it is not surprising that you can get
 better results. The question is whether people are willing to work with
 such more complicated models in their force fields. It is certainly
 worthwhile exploring in which situations such models can give better
 results.
 --
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