  We simulate the homogeneous nucleation of ice from supercooled liquid water
at 220 K in the isobaric-isothermal ensemble using the MW monatomic water
potential. Monte Carlo simulations using umbrella sampling are performed in
order to determine the nucleation free energy barrier. We find the Gibbs energy
profile to be relatively consistent with that predicted by classical nucleation
theory; the free energy barrier to nucleation was determined to be ~18 kT and
the critical nucleus comprised ~85 ice particles. Growth from the supercooled
liquid gives clusters that are predominantly cubic, whilst starting with a
pre-formed subcritical nucleus of cubic or hexagonal ice results in the growth
of predominantly that phase of ice only.
