Résumé
The study of small/medium size molecules inside nanoscale cavities of diverse host material, e.g., clathrates hydrates, cryogenic fluids (or superfluids), fullerenes, carbon nanotubes and zeolites, has receiveda great deal of attention over the past years due to their broad application domain (condensed matter physics,nanomaterial sciences, geoscience, quantum chemistry, astrophysical and planetary sciences, biophysics, ...).However the description of such systems is often far from complete. Indeed,in such nanoscale confinement, the translational center-of-mass motionsof the caged molecules are quantized and strongly coupled to themolecular rotations, which are quantized too. In this study, we examine the effect of a flexible description of the clathrate hydrate framework on the translation-rotation (TR) eigenstates of guest molecules such as molecular hydrogen. Traditionally, the water cage structure is assumed to be rigid, thus ignoring the quantum nature of hydrogen nuclei in the water framework. 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 decoupling scheme. Our approach combines rigid body Diffusion Monte Carlo calculations for the description of the rotational degree of freedom of water molecules surrounding the guest molecular hydrogen to an efficient Smolyak sparse-grid technique for the calculation of the TR levels. This approach allows us to take into account the highly anharmonic nature of the rotational water motions in a high-dimensional system. The clathrate-induced splittings of the $j= 1$ rotational levels are much more sensitive to the quantum hydrogen delocalisation than the translational transitions.