  Nucleation in small volumes of water has garnered renewed interest due to the
relevance of pore condensation and freezing under conditions of low partial
pressures of water, such as in the upper troposphere. Molecular simulations can
in principle provide insight on this process at the molecular scale that is
challenging to achieve experimentally. However, there are discrepancies in the
literature as to whether the rate in confined systems is enhanced or suppressed
relative to bulk water at the same temperature and pressure. In this study, we
investigate the extent to which the size of the critical nucleus and the rate
at which it grows in thin films of water are affected by the thickness of the
film. Our results suggest that nucleation remains bulk-like in films that are
barely large enough accommodate a critical nucleus. This conclusion seems
robust to the presence of physical confining boundaries. We also discuss the
difficulties in unambiguously determining homogeneous nucleation rates in
nanoscale systems, owing to the challenges in defining the volume. Our results
suggest any impact on a film's thickness on the rate is largely inconsequential
for present day experiments.
