Free-energy calculation of structure-H hydrates

A molecular-dynamics (MD) simulation of structure-H hydrates was performed under constant pressure and temperature with 6120 TIP4P water molecules, 900 OPLS-UA methane molecules, and 180 large molecular guest substance (LMGS) molecules. The LMGS molecules were represented in the form of a one-site Lennard-Jones (LJ) model using the LJ parameters δ and ε. In order to clarify the thermodynamic stability of structure-H hydrates, we calculated the free-energy difference, changing on the δ and ε only of the LMGS molecules. In this simulation, stable crystals of structure-H hydrates and a minimum value of ΔG were obtained at δ∼6.2 Å and large values of ε. All simulations were performed using the special-purpose computer hardware MDGRAPE-2.