Free-Energy Calculations of the Intercage Hopping Barriers of Hydrogen Molecules in Clathrate Hydrates

Clathrate hydrates are nonstoichiometric crystalline inclusion compounds in which a water host lattice encages small guest atoms or molecules in cavities, and they have potential utility as a hydrogen-storage vector. In spite of the anomalous mechanistic nature of guest-diffusivity in clathrate hydrates, characterizing the precise mechanisms of intercage diffusive migration therein remains an elusive challenge. Also, nuclear quantum effects are particularly important for small guests such as H-2, and cannot realistically be neglected in the host lattice in any rigorous dynamical treatment of H-2 intercage diffusivity. Here we compute free-energy profiles and barriers, showing that quantal delocalization increases these barriers dramatically vis-a-vis classical dynamics for intercage H2 diffusion, by combining umbrella sampling with path-integral molecular dynamics in the extended solid. Results are compared to earlier DFT ab initio molecular dynamics calculations of Trinh et al, who found that the free-energy barriers decrease with increasing temperature.