  Colloidal clusters consist of small numbers of colloidal particles bound by
weak, short-range attractions. The equilibrium probability of observing a
cluster in a particular geometry is well-described by a statistical mechanical
model originally developed for molecules. To explain why this model fits
experimental data so well, we derive the partition function classically, with
no quantum mechanical considerations. Then, by comparing and contrasting the
derivation in particle coordinates with that in center-of-mass coordinates, we
physically interpret the terms in the center-of-mass formulation, which is
equivalent to the high-temperature partition function for molecules. We
discuss, from a purely classical perspective, how and why cluster
characteristics such as the symmetry number, moments of inertia, and
vibrational frequencies affect the equilibrium probabilities.
