Subject: Dipole-constrained ESP charges

From: "Benny G. Johnson" <[email-masked]>
Date: Tue, 8 Mar 1994 14:16:05 -0600

Dr Walter E. Reiher writes:

> I'm trying out MOPAC 93 and have encountered the following unexpected
> result:  when fitting atom-centered point charges using the 'ESP'
> keyword, I find that the charges obtained with the 'DIPOLE' keyword
> (which is supposed to 'fit the ESP to the calculated dipole') give
> charges which physically unreasonable and have a *worse* fit to the
> calculated dipole.

At one point I worked on the ESP code in Gaussian 92, and I believe the
routine which does the dipole-constrained charge fits has the same
origin as the one in MOPAC.  One problem is that for a molecule such as
water, which has a component of the dipole moment equal to zero by symmetry,
the least-squares matrix equation for the dipole-constrained fit is
singular.  This is trivial to fix (just leave out the offending 0 = 0
equation), but the original code did not consider this, and thus one
obtained numerical garbage for the charges from trying to invert a
singular matrix.  Prof. J. Stewart noted in an earlier posting that the
same problem occurred with formaldehyde, which is consistent.

I passed this fix on to some of the people working on MOPAC,
but it is certainly possible the bug still exists in some versions
of the code.  I don't have your original email message, so I don't 
have your address, but if you're interested, write to me and
I'll try to put you in contact with someone who should have a fixed
version of this routine.

Hope this helps,
Benny Johnson
Q-Chem, Inc.