  Langevin equations are used to model many processes of physical interest,
including low-energy nuclear collisions. In this paper we develop a general
method for computing probabilities of very rare events (e.g. small fusion
cross-sections) for processes described by Langevin dynamics. As we demonstrate
with numerical examples as well as an exactly solvable model, our method can
converge to the desired answer at a rate which is orders of magnitude faster
than that achieved with direct simulations of the process in question.
