  The accuracy of rate constants calculated using transition state theory
depends crucially on the correct identification of a recrossing--free dividing
surface. We show here that it is possible to define such optimal dividing
surface in systems with non--Markovian friction. However, a more direct
approach to rate calculation is based on invariant manifolds and avoids the use
of a dividing surface altogether, Using that method we obtain an explicit
expression for the rate of crossing an anharmonic potential barrier. The
excellent performance of our method is illustrated with an application to a
realistic model for LiNC$\rightleftharpoons$LiCN isomerization.
