FULL-COORDINATE FREE-ENERGY MINIMIZATION FOR COMPLEX MOLECULAR-CRYSTALS - TYPE-I HYDRATES

A method is presented for efficient calculation of free energies for m olecular crystals. The method is based on a generalisation of the loca l harmonic approximation. These calculations are rapid enough to allow optimisation of the free energy with respect to all atomic and crysta llographic coordinates at finite temperatures for complex molecular cr ystals. The procedure has been illustrated for methane hydrate, which has about 150 atoms in the unit cell, and ice Ih. The resulting free e nergies of the fully optimised structures have been used to predict th e conditions for three-phase equilibrium between methane hydrate, ice and methane gas, and shown to give results in excellent agreement with experiment.