  For systems that self assemble into finite-sized objects, it is sometimes
convenient to compute the thermodynamics for a small system where a single
assembly can form. However, we show that in the canonical ensemble the use of
small systems can lead to significant finite-size effects due to the
suppression of concentration fluctuations. We introduce methods to estimate the
bulk-yields from simulations of small systems and to follow the convergence of
yields with system size, under the assumptions that the various species behave
ideally. We also propose an extension to the umbrella sampling technique that
allows the formation of multiple finite-sized objects.
