  We propose a hybrid algorithm for the time integration of large sets of rate
equations coupled by a relatively small number of degrees of freedom. A subset
containing fast degrees of freedom evolves deterministically, while the rest of
the variables evolves stochastically. The emphasis is put on the coupling
between the two subsets, in order to achieve both accuracy and efficiency. The
algorithm is tested on the problem of nucleation, growth and coarsening of
clusters of defects in iron, treated by the formalism of cluster dynamics. We
show that it is possible to obtain results indistinguishable from fully
deterministic and fully stochastic calculations, while speeding up
significantly the computations with respect to these two cases.
