  We use molecular simulation to calculate the nucleation free energy barrier
for the freezing of a 456 atom gold cluster over a range of temperatures. The
results show that the embryo of the solid cluster grows at the vapor-surface
interface for all temperatures studied and that the usual classical nucleation
model, with the embryo growing in the core of the cluster, is unable to predict
the shape of the free energy barrier. We use a simple partial wetting model
that treats the crystal as a lens shaped nucleus at the liquid-vapor interface
and find that the line tension plays an important role in the freezing of gold
nanoparticles.
