  A number of crystal structures of water have been `superheated' in Monte
Carlo simulations. Two well known models for water were considered; namely the
TIP4P model and the SPC/E model. By comparing the fluid-solid coexistence
temperature to the temperature at which the solid becomes mechanically unstable
and melts it is possible to determine the typical range of temperatures over
which is possible to superheat the ice phases in conventional simulation
studies. It is found that the ice phases can be superheated to approximately
90K beyond the fluid-solid coexistence temperature. Beyond this limit they
spontaneously melt. This limit appears to depend weakly both on the type of ice
phase considered and on the chosen model. Obviously only rigorous free energy
calculations can determine the equilibrium fluid-solid coexistence of a model.
However, a "rule of thumb" is that, by subtracting 90K from the mechanically
stability limit of the the ice phase one is provided with a first guess as to
the equilibrium fluid-solid coexistence temperature.
