Ab initio calculations at the HF/6-31G* level and experimental data are used to derive a multidimensional analytic potential energy function for the Cl- + CH3Cl SN2 nucleophilic substitution reaction. Each atom and degree of freedom is treated explicitly. This potential gives a SN2 rate constant for Cl- + CH3Cl at 300 K that is within a factor of 2 of the experimental value. Canonical variational-transition-state structures and rate constants are calculated from this potential for the cluster-forming reaction Cl- + CH3Cl → Cl-⋯CH3Cl. Only one variational transition state is found at each temperature. The analytic function presented here has been incorporated into the general chemical dynamics computer program VENUS. This function, when complemented by additional functions available in VENUS, is sufficiently general that it can be used to represent other symmetric and asymmetric SN2 nucleophilic substitution reactions.