Molecular Dynamics Simulation of the Three-Phase Equilibrium Line of CO₂ Hydrate with OPC Water Model

The three-phase coexistence line of the CO₂ hydrate was determined using molecular dynamics (MD) simulations. By using the classical and modified Lorentz-Berthelot (LB) parameters, the simulations were carried out at 10 different pressures from 3 to 500 MPa. For the OPC water model, simulations with the classic and the modified LB parameters both showed negative deviations from the experimental values. For the TIP4P/Ice water model, good agreement with experimental equilibrium data can be achieved when the LB parameter is adjusted based on the solubility of CO₂ in water. Our results also show that the influence of the water model on the equilibrium prediction is much larger than the CO₂ model. Current simulations indicated that the H₂O-H₂O and H₂O-CO₂ cross-interactions' parameters might contribute equally to the accurate prediction of T₃. According to our simulations, the prediction of T₃ values showed relatively higher accuracy while using the combination of TIP4P/Ice water and EPM2 CO₂ with modified LB parameter. Furthermore, varied χ values are recommended for accurate T₃ estimation over a wide pressure range. The knowledge obtained in this study will be helpful for further accurate MD simulation of the process of CO₂/CH₄ replacement.