  Freezing in the vicinity of water-vapor interfaces is of considerable
interest to a wide range of disciplines, most notably the atmospheric sciences.
In this work, we use molecular dynamics and two advanced sampling techniques,
forward flux sampling and umbrella sampling, to study homogeneous nucleation of
ice in free-standing thin films of supercooled water. We use a coarse-grained
monoatomic model of water, known as mW, and we find that in this model a
vapor-liquid interface suppresses crystallization in its vicinity. This
suppression occurs in the vicinity of flat interfaces where no net Laplace
pressure in induced. Our free energy calculations reveal that the pre-critical
crystalline nuclei that emerge near the interface are thermodynamically less
stable than those that emerge in the bulk. We investigate the origin of this
instability by computing the average asphericity of nuclei that form in
different regions of the film, and observe that average asphericity increases
closer to the interface, which is consistent with an increase in the free
energy due to increased surface-to-volume ratios.
