  Chemical reactions on dust grains are of crucial importance in interstellar
chemistry because they produce molecular hydrogen and various organic
molecules. Due to the submicron size of the grains and the low flux, the
surface populations of reactive species are small and strongly fluctuate. Under
these conditions rate equations fail and the master equation is needed for
modeling these reactions. However, the number of equations in the master
equation grows exponentially with the number of reactive species, severely
limiting its feasibility. Here we present a method which dramatically reduces
the number of equations, thus enabling the incorporation of the master equation
in models of interstellar chemistry.
