(192aw) Hydrogen-Hydrate Cage-Hopping: Insights from Molecular Dynamics

Clathrate hydrates have potential utility as an hydrogen-storage vector. In spite of the anomalous mechanistic nature of hydrogen diffusivity in clathrate hydrates, characterising the precise mechanisms of inter-cage diffusive migration remains elusive. Also, nuclear quantum effects are particularly important for small guests such as H₂, and cannot realistically be neglected in the host lattice in any rigorous dynamical treatment of H₂ inter-cage diffusivity. Here, for the 50-200 K range, we compute free-energy profiles as a function of large-cage occupation (single to quintuple), showing that quantal delocalisation increases these barriers dramatically vis-à-vis classical dynamics for inter-cage H₂ diffusion, by combining umbrella sampling with classical and path-integral molecular dynamics in the sII-type extended solid. Results are compared to earlier DFT ab-initio molecular dynamics calculations of Trinh et al (propagated classically), who found that the free-energy barriers decrease with increasing temperature. We also probe spatial localisation within the large cavity as a function of temperature and occupation. We finish with a critique of available intermolecular potentials for treating hydrogen hydrates, as well as probing H₂ vibrational frequencies within hydrate cavities, drawing insights from, and comparison with, available experimental data and Density Functional Theory results.