Monte Carlo calculations of the free energy of ice-like structures using the self-referential method

The self-referential method is a recently developed technique to compute the free energy of molecular crystals. In this paper, the method is extended to systems composed of nonlinear rigid molecules and applied to obtain the Helmholtz free energy of ice VII, hexagonal ice, cubic ice, and the Gibbs free energy of the empty structure I (sI) clathrate hydrate and fully occupied methane sI. It is shown that the method provides a viable alternative to other techniques to determine the free energy of solids. Good agreement with available reference literature data is found. We expect that the technique can be applied to a wide range of molecular crystals.