We introduce a numerical method based on the level-set technique to compute
capture numbers used in mean-field rate equations that describe epitaxial
growth. In our level-set approach, islands grow with a velocity that is com
puted from solving the diffusion equation for the adatom concentration. The
capture number for each island is then calculated by integrating the growt
h velocity of an island around the island boundary. Thus, our method by con
struction includes all spatial correlations between islands. The functional
form of the capture numbers a, is, to first approximation, affinely depend
ent on the island sizes. Integration of a completely deterministic set of m
ean-field rate equations for the first time properly reproduces the correct
island densities and cluster size distribution.