Subject: popln. analysis and dipoles

From: [email-masked]
Date: Fri, 3 Dec 1993 12:52 EST

Patrick B.'s comments on population analysis and dipole
moments strike a cord.  I have heard these complaints(and
made some myself) for a number of years.
1)  the choice of divison of the overlap population is
arbitrary... various lines of thought have gone into the
derivation of literally hundreds of methods of doing this
but the bottem line is that you do not have a quantum mechanical
operator for the population in an 'orbital' on a particular
atom that 'belongs' to that atom. Perhaps the most attractive
methods are those that arise through the use of quantum
mechanics, do not use arbitrary boundaries and as well
apply to the total density. (eg. Richard Bader et al's
theory of mol. structure)
2) the 'charge' on an atom is not a point charge, this is
a gross assumption that in itself can lead to large errors.
It appears to arise from the conceptually appealing picture
of nice spherical atoms in molecules. The density
is, however, not spherical around each atom in a molecule. Thus,
using a point charge model with populations calculated through
arbitrary division of the overlap populations has a good chance
of giving poor results.
  The above being said, these different methods of treating
the problem of population analysis has led to a lot of chemical
insight for various problems. You must keep the weaknesses in
mind when choosing an approach.

 Any other thoughts on this?
                            Ken Edgecombe

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