Subject: ESP charges fit to dipole in MOPAC93

From: [email-masked] (Walter E. Reiher III)
Date: Mon, 7 Mar 1994 15:19:02 -0800 (PST)

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.

My test case is water, experimental geometry, and I made two runs
including the following keywords:
     AM1  AIGIN  MULLIK  +
     ESP CONNOLLY NSURF=4 SCALE=1.4 SCINCR=0.2 SYMAVG
MOPAC 93 gave me ESP-fit charges of 0.2279 and -0.4557, giving a 'dipole
computed from point charges' of 1.2828.  The computed dipole moment was
good, 1.854, so I thought I would try biasing the fit to the calculated
dipole.

In my second run, I added the keyword 'DIPOLE' to the above set.  MOPAC
93 gave me H's more charged than the O and charges of opposite sign on
the H's!
            ELECTROSTATIC POTENTIAL CHARGES
            DIPOLE CONSTRAINTS WILL BE USED
               ATOM NO.    TYPE    CHARGE
                  1          O    -0.0013
                  2          H    -0.0480
                  3          H     0.0173
Besides this, the dipole moment is too small.  It can't be just a
scaling error, since the H's are so different.

I scanned the CCL archives and couldn't find anything on this.  Anyone
have an idea?  Thanks.

Wally
========================================================================
Walter E. Reiher III, Ph.D.                            [email-masked]
Consultant in Computational Chemistry                 voice 408-720-0240
P.O. Box 61056                                          fax 408-720-0378
Sunnyvale, CA 94088                        24-hr voice mail 408-534-1171