Rare events via multiple reaction channels sampled by path replica exchange.

Transition path sampling (TPS) was developed for studying activated processes in complex systems with unknown reaction coordinate. Transition interface sampling (TIS) allows efficient evaluation of the rate constants. However, when the transition can occur via more than one reaction channel separated by a high barrier, TPS and TIS are ineffective in sampling both channels. The combination of replica exchange with TIS can overcome this problem. This work shows how, by including both the backward and forward reactions, the corresponding rate constants, as well as the free energy barrier can be computed in a single simulation. The method is illustrated on a two dimensional potential using the Langevin dynamics. In addition, a simpler algorithm based on only forward shooting from the interfaces is shown to give equally accurate results, and forms a bridge between the transition interface and the forward flux sampling methods. The diffusive behavior of the replicas can be used to assess the quality of the choice of the order parameter used for the interfaces.