is the probabilty that a trajectory crosses the product interface 𝜆 𝑛 , when starting from reactant interface 𝜆 0 . In order to improve the efficiency of the computational evaluation of 𝑃 (𝜆 𝑛 |𝜆 0 ), further developments of the TIS algorithm, such as replica exchange TIS (RETIS ), have been proposed, introducing exchange moves between interface ensembles in order to enhance the ergodicity of the sampling. As for TPS, in TIS the CV 𝜆 is used to conveniently partition configuration space, and does not affect the sampling of reactive trajectories.