  We investigate the contraction of accreting protoclusters using an extension
of n-body techniques that incorporates the accretional growth of stars from the
gaseous reservoir in which they are embedded. Following on from Monte Carlo
studies by Davis et al., we target our experiments toward populous clusters
likely to experience collisions as a result of accretion-driven contraction. We
verify that in less extreme star forming environments, similar to Orion, the
stellar density is low enough that collisions are unimportant, but that
conditions suitable for stellar collisions are much more easily satisfied in
large-n clusters, i.e. n ~ 30,000 (we argue, however, that the density of the
Arches cluster is insufficient for us to expect stellar collisions to have
occurred in the cluster's prior evolution). We find that the character of the
collision process is not such that it is a route toward smoothly filling the
top end of the mass spectrum. Instead, runaway growth of one or two extreme
objects can occur within less than 1 Myr after accretion is shut off, resulting
in a few objects with masses several times the maximum reached by accretion.
The rapid formation of these objects is due to not just the post-formation
dynamical evolution of the clusters, but an interplay of dynamics and the
accretional growth of the stars. We find that accretion-driven cluster
shrinkage results in a distribution of gas and stars that offsets the
disruptive effect of gas expulsion, and we propose that the process can lead to
massive binaries and early mass segregation in star clusters.
