Subject: Re: CCL:Merz-Kollman charges - constraints

From: Jacco van de Streek <[email-masked]>
Date: Fri, 26 Jul 2002 09:12:07 +0100

Marcin Krol wrote:
> 
> Dear CCL'ers
> 
> I'm using MK charge fitting scheme as impl;emented in Gaussian98 for a
> symmteric molecule. However, charges, which should be equivalent, differ
> for about 0.1e. Increasing number of points doesn't help. Do you know of
> any program which would constrain charges on equivalent atoms before the
> fit (like in RESP or CHELP-BOW methods)? Or maybe Gaussian can do it?

In my experience with Gaussian94, using the Breneman CHelpG instead of the
Merz-Kollman set of sampling points solves the problem (replace pop=MK with
pop=CHelpG). I think this has to do not only with the number of points, but also
with the way they are chosen.

Constraining the atomic charges to reproduce the QM dipole moment will also
help: pop=(CHelpG,dipole).

Neither of these suggestions mathematically constrains charges on
symmetry-equivalent atoms, but I have found that in practice the charges on
symmetry-equivalent atoms are usually identical or nearly identical (within,
say, 0.01 e).

Switching from MK to CHelpG has the added advantage (in Gaussian94) that
Gaussian will not crash if you ever need the ReadRadii key word (for elements
for which no Van der Waals radii are stored within Gaussian, like bromine).

Best wishes,
-- 
Jacco van de Streek
Research Scientist
Cambridge Crystallographic Data Centre
Cambridge
United Kingdom