The symmetry number of a molecule is obtained by imagining all identical atoms to
be labelled, and then counting the number of different but equivalent arrangements
that can be obtained by rotating (but not reflecting) the molecule. In the statistical-mechanical
treatment of
chemical equilibrium, the
partition function for each molecular species must be divided by its symmetry number.
Source:
PAC, 1996, 68, 149
(A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC
Recommendations 1996))
on page 189
