A factor involving activity coefficients that appears as a multiplier in the rate
equation. For a bimolecular reaction the factor is

,

where , and are the activity coefficients for the reactants A and B and the activated complex, respectively. For a unimolecular reaction the factor is
,

and for a trimolecular reaction it is
.

Source:

PAC, 1996, *68*, 149*
(A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC
Recommendations 1996))
* on page 171