From a Liquid to a Crystal without Going through a First-Order Phase Transition: Determining the Free Energy of Melting with Glassy Intermediates

The excess free energy of a liquid relative to an Einstein crystal reference state is calculated without going through a first-order phase transition. This is accomplished by going through an arrested glassy state to avoid a direct liquid to gas or liquid to crystal transition. The method is demonstrated by calculating the free energy difference between liquid water and ice Ih using the TIP4P and WAIL water models. TIP4P ice Ih melts at 232 +/- 1 K, in close agreement with other estimates in the literature. WAIL ice melts at 272 +/- 1 K, in good agreement with that of real water, which serves as a good validation of the quality of the WAIL model. The glassy intermediate method is easy to implement and amicable to parallel executions. We expect this method to have broad applications for calculating the liquid excess free energies for other materials.