  We apply quantum rate theory to calculate the transition rates as hydrogen or
deuterium atoms escape from a vacancy trap in iron into a neighbouring
metastable site. We determine transition rates and corresponding activation
energies over a wide range of temperatures covering both the quantum and
classically dominated regimes. We find that quantum effects lead to an increase
of the transition rate activation energy and to very significant recrossing of
the transition state dividing surface. As a result of recrossing quantum
transition state theory overestimates the rate of proton transfer by more than
an order of magnitude and the rate of deuteron transfer by a factor of two.
