  Back-diffusion is the phenomenon by which random walkers revisit binding
sites on a lattice. This phenomenon must occur on interstellar dust particles,
slowing down dust-grain reactions, but it is not accounted for by standard
rate-equation models. Microscopic kinetic Monte Carlo models have been used to
investigate the effect of back-diffusion on reaction rates on interstellar dust
grains. Grain morphology, size, and grain-surface coverage were varied and the
effects of these variations on the magnitude of the back-diffusion effect were
studied for the simple H+H reaction system. This back-diffusion effect is seen
to reduce reaction rates by a maximum factor of ~5 for the canonical grain of
10$^6$ binding sites.The resulting data were fit to logarithmic functions that
can be used to reproduce the effects of back-diffusion in rate-equation models.
