Subject: Population analysis and dipoles

From: Anthony Stone <[email-masked]>
Date: Mon, 6 Dec 1993 10:12:05 +0000 (GMT)

Population analysis and dipole moments

The dipole moment of a molecule can be expressed in terms of atomic moments
as sum (atom charge)*(position) + sum (atomic dipoles). This is exact if you
define things correctly, but the atomic dipole values will depend on your
choice of definition for the atomic charges. The atomic dipoles are usually
significant, whatever definition you use for the atom charges, so if you leave
them out you will get rotten molecular dipole moments. An exception to this is
potential-fitted point charges, which give quite good values for the molecular
dipole, but they achieve this by representing atomic dipoles by charges on
nearby atoms, so the potential isn't very good in the neighbourhood of a
dipolar atom.

The Distributed Multipole Analysis procedure, described by Stone & Alderton
in Mol. Phys. 110, 123 (1984), generates charges, dipoles, etc. from a
wavefunction in a consistent manner (i.e. so that the overall molecular
moments are correct for the wavefunction), and in such a way that the higher
atomic moments are as small as possible. It is included in the Cadpac
program.  Bader's partitioning, mentioned by Ken Edgecombe, is much more
elegant but it leads to much larger high-rank moments, which is an
inconvenience for most applications.

For a more general discussion see Stone & Price, J. Phys. Chem. 92, 3325
(1988).

Anthony Stone
University Chemical Laboratory,         Internet: [email-masked]
Lensfield Road,                         Phone:    +44 223 336375
Cambridge CB2 1EW, U.K.                 Fax:      +44 223 336362