Subject: ESP in MOPAC

From: [email-masked] (Dr. James Stewart)
Date: Tue, 8 Mar 94 09:37:34 PST

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.

After running the test-system (water) I confirm that there is something
odd going on in the ESP calculation.  The ESP calculation was a first
attempt at charge-fitting.  A check of the test-data supplied with the
original ESP code shows that the algorithm is working correctly. Therefore,
the obvious conclusion is that the ESP method has difficulty with water.
A check on formaldehyde shows that problems also exists with at least one
simple organic molecule.

Very recently, Prof George Ford and Dr B. Wang have written a much faster 
ESP calculation, which they call PMEP (Parametric Molecular Electrostatic 
Potential), and using their method the results for water and formaldehyde
are very reasonable.

Because the Ford-Wang method is much faster, simpler, and easier to use,
it will replace the older ESP method in MOPAC.  

It is important that no criticism should be levelled at the authors of the
original ESP method used in MOPAC 6. Users of these methods 
should understand that a new technique, such as the original ESP, represent
a pioneering effort, and that inevitably defects in the new method will be
found.  Further progress is made both when defects or limitations are 
found and when improvements are made to existing methods.  

Based on this concept, Dr Reiher and Drs Ford and Wang have contributed
beneficially to the development of semiempirical methods, and I thank
them for their input.

James J. P. Stewart