Mechanisms for thermal conduction in various polymorphs of methane hydrate

Extensive equilibrium molecular-dynamics simulations have been performed to investigate thermal-conduction mechanisms via the Green-Kubo approach for fully occupied type I, II, and H methane hydrates, in addition to ice Ih and a hypothetical empty type I hydrate structure. The TIP4P water model was used in conjunction with a fully atomistic methane potential with which it had been parameterized from quantum simulation, along with long-range Ewald electrostatics. We have found that the crystal structure of the clathrate framework and guest-host interactions in type I methane hydrate contribute to a lower thermal conductivity vis-\`a-vis ice Ih and its glasslike temperature dependence, respectively; damping in methane-host energy transfer above 100 K was determined to be responsible for the latter. However, we have found that substantially less damping in guest-host energy transfer is present in type II and H methane-hydrate polymorphs at higher temperatures, giving rise to somewhat larger thermal conductivities relative to type I methane hydrate with a crystal-like temperature dependence.