Efficient Determination of Water/Ice Phase Diagram through Isenthalpic–Isobaric Molecular Dynamics Simulations

Predicting the water phase diagram is a powerful way to evaluate water models through molecular simulations. Equilibrium points are usually obtained through free energy calculations or direct coexistence simulations in the <i>NPT</i> ensemble. The former can be complex, especially for ice with partial proton order, while the latter can require multiple long and computationally costly simulations. In this work, we report the melting points of ice Ih, III, V, and VI between 0.1 and 1190 MPa through molecular dynamics direct coexistence simulations in the <i>NPH</i> ensemble. Our results are consistent with the original TIP4P/Ice work coexistence lines, except for ice III, for which we report a much larger stability region. Our data agree with recent works, validating this methodology as an alternative to free energy calculations and <i>NPT</i> direct coexistence simulations for high-pressure phases of ice.