Subject: RE: CHELPG

From: "Shobe, Dave" <[email-masked]>
Date: Tue, 09 Jul 2002 15:11:18 -0400

Mulliken charges are assigned to atoms based on the "usage" of the atoms'
basis functions in the molecular orbitals that are calculated.  It is known
as a quick-and-dirty way of assigning charges, but one that doesn't
necessarily have any physical meaning.  Mulliken charges are notoriously
basis-set dependent.  

CHELPG charges are based on the electrostatic field generated by the
molecule.  It is a best-fit method; the set of charges produced by the
CHELPG method are the point charges that would produce an electric field
which is as similar as possible to the molecule's.  Accordingly, if you need
the charges for molecular-mechanics calculations which include electrostatic
interactions, CHELPG is arguably the best for that purpose.

There are other methods of assigning atomic charges.  GAPT charges are based
on the derivative of the molecular dipole moment WRT the atomic positions.
AIM charges are based on the division of 3-dimensional space among the atoms
in the molecule, according to a surface that has certain mathematical
properties.  NBO charges are similar to Mulliken charges, but the orbitals
are transformed into "natural" orbitals, which fixes some of the basis-set
dependency of the Mulliken method.  

--David Shobe
Süd-Chemie Inc.
phone (502) 634-7409
fax     (502) 634-7724
email  [email-masked]

Don't bother flaming me: I'm behind a firewall.

-----Original Message-----
From: George Hall [mailto:[email-masked]]
Sent: Tuesday, July 09, 2002 10:21 AM
To: [email-masked]
Subject: CCL:CHELPG

Hi everybody,

I wonder if some could point me out what are the differences between Chelpg 
charges and Mulliken.

Would you please indicate me some references or some advices about the 
aproach for these kind of charges.

Thanks a lot in advance.

George Hall.

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