The random motion of large, two-dimensional adatom clusters and vacanc
y clusters over a metal surface is described by a diffusion coefficien
t D proportional to d(-n), where d is the diameter of the cluster and
the integer n identifies the diffusion mechanism. For circular cluster
s, n = 3 when center-of-mass motion occurs by adatom diffusion along t
he periphery of the cluster, while n = 2 or 1 when cluster diffusion o
ccurs by correlated or uncorrelated adatom evaporation and condensatio
n, respectively. A faceted adatom cluster diffuses by evaporation and
condensation of facet ledges, giving n = 0 when cluster diffusion is d
etermined by the rate at which erosion of a ledge is initiated.