Prediction of hydrogen hydrate equilibrium by integrating ab initio calculations with statistical thermodynamics

This paper addresses a new calculation approach for the prediction of hydrogen hydrate equilibrium by introducing the concept of a single hydrogen cluster in one cavity. By integrating ab initio calculations with classical statistical thermodynamics, this approach enables the van der Waals model to predict the dissociation pressure of hydrogen hydrates. Compared to hydrates formed by light hydrocarbon gases, structure II (sII) hydrogen hydrates stably encage two and four hydrogen molecules in the small and large cavities, respectively. By treating two hydrogen molecules or four hydrogen molecules as one rigid body cluster, we determine ab initio binding energies between water molecules and hydrogen clusters at the MP2 level with the 6-31 ++G(2d,2p) basis set. These binding energies will be used to determine the parameters of the Exp-6 potential function from which the smooth cell potential and the Langmuir constant of each cluster are calculated. Then, the dissociation pressure is determined using the Zele-Lee-Holder cell distortion model: 105, 625, and 2000 bar at 150, 200, and 250 K, respectively.