Subject: CCL:M:Charges and dipole moments (fwd)

From: [email-masked] (Christopher Cramer)
Date: Thu, 25 Sep 1997 14:41:01 -0500 (CDT)

Re below discussion on charges:
> > 
> > How are the default charges computed in MOPAC?  I understand how the
> > Mulliken charges are obtained, but I have never seen a reference to the
> > default charges.  I am assuming they are obtained by calculating the charge
> > density from the localized MO's.  Is this correct?  
> > 
> > Often Mulliken charges do not reproduce physical observables such as dipole
> > moments accurately.  Are there any references to alternative methods which
> > calculate atomic charges and show good agreement with experimentally
> > determined dipole moments?  Thank you in advance.
> Some alternative charge distributions:
> 
> * CHELPG (charges from electrostatic potentials): C. M. Breneman and K. B.
> Wiberg, {\it J. Comput. Chem.} {\bf 11}, 361 (1990); see also Chirlian and
> Francl, quoted there. This should be the best for getting dipole moments
> from the partial charges close to the one obtained from the wave function.
> (Agreement with expt. cannot be any better than for the underlying wave 
> function, except fortuitously.)
> 
   Quite the contrary -- a charge model can do better than the underlying
wave function if it is DESIGNED to do so (what we have coined a Class IV
charge model). The CM1 charge model (reference Storer, J. W.; Giesen, D.
J.; Cramer, C. J.; Truhlar, D. G. " Class IV Charge Models: A New
Semiempirical Approach in Quantum Chemistry" J. Comput.-Aid. Mol. Des.
1995, 9, 87. available on the web at
http://www.ibc.wustl.edu/jcamd/278/paper.html) is a mapping procedure that
takes lower quality charges (e.g., semiempirical ZDO Mulliken) and converts
them to very high-quality charges that reproduce experiimental dipole
moments better than the expectation value of the dipole moment operator
acting on the semiempirical wave function.

CJC

-- 

Christopher J. Cramer
University of Minnesota
Department of Chemistry
207 Pleasant St. SE
Minneapolis, MN 55455-0431
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[email-masked]
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