Subject: POPULATION ANALYSIS

From: Patrick Bultinck <[email-masked]>
Date: Fri, 3 Dec 1993 16:14:35 +0100 (MET)

I have performed some population analysis calculations on covalent
molecules using ab initio Hartree-Fock. I performed MPA and LPA (mulliken
and lowdin). The results were often miles apart. The reason, I think, is
the fact that every value for n in S^nPS^(1-n) will do for defining a
population analysis formula.
I do no longer believe very much in the use of population analysis for
determining atomic charges for atoms in a certain molecule. This
disbelieve became bigger when last year, during my graduation thesis, I
studied calculations of dipole moments. Indeed I remembered the formula
DIP. MOM.=sum(atomic charge*R), sum over all atoms. I thought I could see how
good PA was by using the PA results to determine the dipole moment (apart
>from the one calculated with the wavefunction). I calculated the different
cartesian components of the DM, since PA gave me the charges and I knew
>from my input what the cart. coordinates of the atoms were. By putting
together the cart. comp. of the DM I got to a value of the total dipole
moment. Needless to say that the value was sometimes miles apart from
experiment and from the wave-function derived dipole moment...(using RHF
and RHF defined PA formulas)

My question is : 
What is the use of the S^nPS^(1-n) derived population
analysis ?
Is there something crazy about my argumentation to say that PA is not to
be trusted very much ? (esp. where I used the formula for the DM using the
PA results...)

Thanks for replies....

Patrick Bultinck
University of Ghent
E-Mail : [email-masked]