Abstract : 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. How-
ever, it has been shown that protons in a water molecule possess a marked delocalised
character in many situations, ranging from water clusters to proton transfer in the
bulk. In the case of water clathrates, all previous TR bound-state calculations of guest
molecules consider that the caging water molecules are fixed at their equilibrium ge-
ometry. Only recently, a static investigation of the role of proton configurations was
performed by Baˇci ́c and co-workers by sampling a very large number of different
static 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 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 sensi-
tive to the quantum hydrogen delocalisation than the translational transitions. This
results is in good agreement with the previous static study of Baˇci ́c and co-workers.
