Grote-Hynes Theory, Recrossing, and Dynamics of the Committor in Ion-Pair Dissociation

For chemical reactions in a solvent, friction emerges as a consequence of projecting multidimensional dynamics onto the dynamics of a single reaction coordinate. Friction causes recrossing of the dividing surface which can either be accounted for with a transmission coefficient or variationally minimized by optimizing the dividing surface. From a perfect non-recrossing dividing surface, all trajectories would commit to opposite basins in forward and backward time, transition state theory would become exact, the transmission coefficient would become one, and the committor distribution would become perfectly focused at ½. For Na⁺Cl^– dissociation in TIP3P water, we show that recrossing persists even when the ½-committor surface itself is used as the dividing surface. We provide evidence that recrossing cannot be fully eliminated from the dynamics for any configurational coordinate. Consistent with this finding, inertial likelihood maximization finds a combination of ion-pair distance and two solvent coordinates that improves the committor distribution and increases the transmission coefficient relative to those for ion-pair distance alone, but recrossing is not entirely eliminated. Free energy surfaces for the coordinates identified by inertial likelihood maximization show that the solvent friction stems from anharmonicity and shallow intermediates that remain after dimensionality reduction to the dynamically important variables.