The performance of OPC water model in prediction of the phase equilibria of methane hydrate.

Abstract

Molecular dynamics (MD) simulations were performed to determine the three-phase coexistence line of sI methane hydrates. The MD simulations were carried out at four different pressures (4, 10, 40, and 100 MPa) by using the direct phase coexistence method. In current simulations, water was described by either TIP4P/Ice or "optimal" point charge (OPC) models and methane was described as a simple Lennard-Jones interaction site. Lorentz-Berthelot (LB) combining rules were used to calculate the parameters of the cross interactions. For the OPC model, positive deviations from the energetic LB rule were also considered based on the solubility of methane in water. For the TIP4P/Ice water model, the obtained three phase coexistence temperatures showed good agreement with experiment data at higher pressures, which is consistent with previous predictions. For the OPC water model, simulations using the classic and the modified LB parameters both showed negative deviations to the experimental values. Our results also indicated that the deviation of the T3 prediction by the OPC model was not closely correlated with the predicted melting point of ice. At 4 MPa, the modified OPC model showed a better prediction of hydrate equilibrium temperature, even better than the prediction by TIP4P/Ice. Considering the relatively higher accuracy in biomolecular MD of the OPC model, it is suggested that this model may have a better performance in hydrate MD simulations of biomolecule-based additives.