Molecular Simulation of the Phase Diagram of Methane Hydrate: Free Energy Calculations, Direct Coexistence Method, and Hyperparallel Tempering

Different molecular
simulation strategies are used to assess the
stability of methane hydrate under various temperature and pressure
conditions. First, using two water molecular models, free energy calculations
consisting of the Einstein molecule approach in combination with semigrand
Monte Carlo simulations are used to determine the pressure–temperature
phase diagram of methane hydrate. With these calculations, we also
estimate the chemical potentials of water and methane and methane
occupancy at coexistence. Second, we also consider two other advanced
molecular simulation techniques that allow probing the phase diagram
of methane hydrate: the direct coexistence method in the Grand Canonical
ensemble and the hyperparallel tempering Monte Carlo method. These
two direct techniques are found to provide stability conditions that
are consistent with the pressure–temperature phase diagram
obtained using rigorous free energy calculations. The phase diagram
obtained in this work, which is found to be consistent with previous
simulation studies, is close to its experimental counterpart provided
the TIP4P/Ice model is used to describe the water molecule.