Cage occupancy of methane hydrates from Gibbs ensemble Monte Carlo simulations

Isobaric-isothermal Gibbs ensemble Monte Carlo simulations of sI methane hydrates in equilibrium with bulk methane are performed to calculate large and small cage occupancies. The OPLS united-atom Lennard-Jones potential, and a variation, are used to model methane, while the TIP4P/Ice model represents water. This model system produces systematically higher total cage occupancies than those of simulations by Henley and Lucia [1] using TIP4P-Ew water and TraPPE-UA methane. We also see higher total occupancies than those observed in prior Raman spectrometry studies by Uchida et al. [2], but find good agreement for large cage occupancies. The simulations provide comprehensive predictions of large and small cage occupancies for temperatures and pressures within the ranges 270-290 K and 20-400 bar, respectively. These predictions demonstrate the advantages of the Gibbs ensemble for the simulation of clathrate hydrates; the ability to control the temperature and pressure, while not constraining the chemical composition of the hydrate phase is the key advantage of this ensemble.