Ice Ih vs. ice III along the homogeneous nucleation line

Pure water can be substantially supercooled below the melting temperature. The pressures and temperatures where water is observed to freeze in supercooling experiments define the so-called homogeneous nucleation line. The slope of such a line changes from negative to positive as pressure increases. In our work, we use computer simulations with the TIP4P/Ice water model to investigate such a change of slope. The simulation prediction for the melting and the homogeneous nucleation lines is in reasonable agreement with the experiment. We find, in accordance with a previous hypothesis, that the aforementioned slope sign inversion can be explained by a change in the nucleating ice polymorph from ice Ih to ice III. Moreover, we underline the kinetic character of the homogeneous nucleation line by defining different lines for different values of the nucleation rate. We analyse in detail the factors that affect the competition between ices Ih and III in the framework of classical nucleation theory.