  We study the phase equilibrium between liquid water and ice Ih modeled by the
TIP4P/Ice interatomic potential using enhanced sampling molecular dynamics
simulations. Our approach is based on the calculation of ice Ih-liquid free
energy differences from simulations that visit reversibly both phases. The
reversible interconversion is achieved by introducing a static bias potential
as a function of an order parameter. The order parameter was tailored to
crystallize the hexagonal diamond structure of oxygen in ice Ih. We analyze the
effect of the system size on the ice Ih-liquid free energy differences and we
obtain a melting temperature of 270 K in the thermodynamic limit. This result
is in agreement with estimates from thermodynamic integration (272 K) and
coexistence simulations (270 K). Since the order parameter does not include
information about the coordinates of the protons, the spontaneously formed
solid configurations contain proton disorder as expected for ice Ih.
