Subject: None

From: "Don Gregory" <[email-masked]>
Date: 1 Oct 1994 08:36:47 +0000

Subject:                               Time:8:27 AM
  OFFICE MEMO          None                                   Date:10/1/94
In light of Dr. Andrey Bliznyuk's mention regarding empirical
adjustments of charges to fit ab inition interaction energies:
   "It seems, that people forgot to mention one more approach to 
    the problem: fitting charges to reproduce ab-initio energy of
    intermolecular interactions."
I thought I'd expand upon, and possibly clarify the earlier notes 
from MSI by Dr.s Rone and Czerminsky with the following, short,
description of how charges are optimized within the MSI
Quanta4.0/CHARMm22 force-field.

MSI's CHARMm charges are generated much as the same way as
Harvard's approach, i.e. high level ab initio calculations
are used to determine the ESP charges as starting points for
the point charges being placed on nuclear centers.
Then, a number of ab initio "interaction energies" between
the solute/model and water molecules are determined, where the 
waters have been placed at various locations around the solute  
molecule.  These ab initio interaction energies are then used
as "observables" in adjustments of the initial charges obtained
in the early stages, so that the empirical, non-bonded energies
compare as best as possible, with those ab initio 
interaction energies.

Don Gregory
Mgr. Application Science
Molecular Simulations Inc.
([email-masked])