Subject: Charges and dipole moments

From: [email-masked] (mario citra)
Date: Thu, 25 Sep 1997 08:41:40 -0400

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.
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* Mario J. Citra  PhD		 *
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From: "Jan M.L. Martin" <[email-masked]>
Date: Thu, 25 Sep 1997 17:46:23 +0200 (IST)

> 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.)

Other methods include:

* topological method: R. F. W. Bader, {\it Atoms in molecules: a quantum
theory} (Oxford University Press, Oxford, 1990). The most "rigorous", but
also the most computationally expensive

* APT (atomic polar tensor):  J. Cioslowski, {\it J. Am. Chem. Soc.} {\bf
111}, 8333 (1989). The only one that directly relates to an observable
quantity (IR intensities). Very costly to do for JUST that, but get thrown
in as a freebie with a frequency+intensities calculation, so if you need
the latter anyhow... 

* natural population analysis: A. E. Reed, L. A. Curtiss, and F. Weinhold,
Chem. Rev. 88, 899 (1988). COmputationally quite inexpensive and probably
a good compromise. 

If you were to move on to ab initio and/or DFT methods, Mulliken is the
most sensitive of the lot to the basis set and APT to electron
correlation. For more details see e.g.:

F. De Proft, J. M. L. Martin, and P. Geerlings, ``On the performance of 
density functional methods for describing atomic populations, dipole 
moments and infrared intensities'' Chemical Physics Letters 250, 
393--401 (1996). 

P. Geerlings, F. De Proft, and J. M. L. Martin, ``Density-Functional 
Theory Concepts and Techniques for Studying Molecular Charge 
Distributions and Related Properties'', in Theoretical and Computational 
Chemistry, Vol. 4: Recent developments and applications of modern 
density functional theory (ed. J. Seminario), Elsevier, New York, 1996, 
pp. 773-809. 

Finally, a general review article:

S. M. Bachrach, in {\it Reviews in Computational Chemistry,
Volume V} (ed. K. B. Lipkowitz and D. B. Boyd) (VCH, New York, 1995).

JM
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