Abstract
In this study, we examine the effect of a flexible description of the clathrate hydrateframework on the translation-rotation (TR) eigenstates of guest molecules such asmolecular hydrogen. Traditionally, the water cage structure is assumed to be rigid,thus ignoring the quantum nature of hydrogen nuclei in the water framework. How-ever, it has been shown that protons in a water molecule possess a marked delocalisedcharacter in many situations, ranging from water clusters to proton transfer in thebulk. In the case of water clathrates, all previous TR bound-state calculations of guestmolecules consider that the caging water molecules are fixed at their equilibrium ge-ometry. Only recently, a static investigation of the role of proton configurations wasperformed by Baˇci ́c and co-workers by sampling a very large number of differentstatic structures of water clathrates.Here, we investigate the importance of the rotational degrees of freedom of the wa-ter cage on the TR levels of guest molecule using an efficient adiabatic decouplingscheme. Our approach combines rigid body Diffusion Monte Carlo calculations forthe description of the rotational degree of freedom of water molecules surroundingthe guest molecular hydrogen to an efficient Smolyak sparse-grid technique for thecalculation of the TR levels. This approach allows us to take into account the highlyanharmonic nature of the rotational water motions in a high-dimensional system.The clathrate-induced splittings of the j= 1 rotational levels are much more sensi-tive to the quantum hydrogen delocalisation than the translational transitions. Thisresults is in good agreement with the previous static study of Baˇci ́c and co-workers.