  We study the prototypical SN2 reaction Cl$^-$ + CH$_3$Cl $\to$ CH$_3$Cl +
Cl$^-$ in water using quantum mechanics / molecular mechanics (QM/MM) computer
simulations with transition path sampling and inertial likelihood maximization.
We have identified a new solvent coordinate to complement the original
atom-exchange coordinate used in the classic analysis by Chandrasekhar, Smith,
and Jorgensen [Ref1]. The new solvent coordinate quantifies instantaneous
solvent induced polarization relative to the equilibrium average charge density
at each point along the reaction pathway. On the basis of likelihood scores and
committor distributions, the new solvent coordinate improves upon the
description of solvent dynamical effects relative to previously proposed
solvent coordinates. However, it does not increase the transmission coefficient
or the accuracy of a transition state theory rate calculation.
