Free energy calculations on systems of rigid molecules: An application to the TIP4P model of H2O

Free energy calculations of different phases are necessary to establish the thermodynamically stable phase in simulations. A new method is proposed to calculate the free energy of a crystal of rigid molecules, which is slightly different from the method [L. A. Baez and P. Clancy (Mol. Phys. 86, 385, (1995)]. The new method is applied to the ice phase of the TIP4P model for H2O [W. L. Jorgensen, J. Chandrasekhar, J. D. Madura, R. W. Impey, and M. L. Klein, J. Chem. Phys. 79, 926 (1983)]. The free energy of the liquid and gas phase are calculated as well, using different methods as the Widom method, overlapping distribution method, and thermodynamic integration. The melting point of the proton ordered ice Ih of the TIP4P model at atmospheric pressure is found to be Tm=214(±6) K and the boiling point Tb=363(±3) K.