  We present the results of over 90 tight-binding molecular-dynamics
simulations of collisions between three- and five-atom silicon clusters, at a
system temperature of 2000K. Much the most likely products are found to be two
'magic' four-atom clusters. We show that previous studies, which focused on the
equilibrium binding energies of clusters of different sizes, are of limited
relevance, and introduce a new effective binding energy which incorporates the
highly anharmonic dynamics of the clusters. The inclusion of dynamics enhances
the magic nature of both Si4 and Si6 and destroys that of Si7.
