## 1 Introduction  
Hydrogen is considered to be the ultimate alternative clean fuel to supplant the use of hydrocarbons in many contexts. Given its very high energy density, it could also serve as a temporary medium for the storage and transport of energy that can seamlessly integrate into renewable energy applications. Despite its very high energy density, rendering it a tempting alternative, its relatively low volumetric density (in gaseous form) and associated hazards, makes finding a safe and affordable hosting/storage material with high capacity a 'Holy Grail'. Although various candidate materials may appear to challenge these material requirements, most of the proposed solutions are bedevilled with a high manufacturing cost or insufficiently low capacity, thus ruling them out of contention [1]. Clathrate hydrates are non-stoichiometric inclusion compounds in which a host water lattice entraps gas molecules [2]. Clathrate hydrates offer a potentially interesting storage medium for hydrogen--a means for 'green mass storage' [2]. The prospect of using gas hydrates as a cheap and green alternative to metal-organic frameworks, zeolites and hydrides for hydrogen storage to facilitate the green and hydrogen economies is certainly attractive [2]. Although pure hydrogen hydrates (consisting only of hydrogen in sII clathrate cages) have been realised experimentally, firstly by [PERSON] et al. [3, 4], they require very high pressures (around 1.5-2 kbar) to be structurally stable at near-ambient temperatures [2, 3, 4, 5] and are less practical for engineering applications [2, 5, 6]. The maximum theoretical hydrogen capacity of the hydrogen hydrate is 5.3 wt.% (for sII hydrates) at significantly elevated pressures of the order of 2-3 kbar for this higher level of cage-filling, and temperatures of around 250 K [7, 8, 9, 10]. However, this maximal storage capacity of circa 5 wt.% is reliant on double and quadruple occupation of small (5\({}^{12}\)) and large (5\({}^{12}\)6\({}^{4}\)) sII-hydrate cages, respectively. This is a somewhat controversial interpretation of high-pressure neutron-diffraction data. More realistic (lower) cage occupancies have been proposed (e.g., closer to 1 for small cages and typically no more than 2 or sometimes 3 in the large cage [2, 11]), leveraging sophisticated ab-initio molecular-dynamics simulation to assess cage occupancies systematically [11]. Of course, the high-pressure requirements to afford stability of pure hydrogen and realise a larger hydrogen-bearing capacity approaching United States Department of Energy (DOE) goals [5; 6] makes it somewhat of an impractical hydrogen-storage medium [1] despite interesting 'ice-coating' proposals [12]. A pragmatic approach for engineering applications hinges on the use of a mixing gas for binary hydrates, in which a 'helper' molecule fills (most) large cavities and the smaller cages are free to host hydrogen. In this respect, both propane [13; 14] and tetrahydrofuran (THF) hydrate have been advanced as potentially attractive propositions [15]. Such 'doping', however, does tend to undermine the hydrogen-bearing hydrates' green' credentials, and must be considered in the appraisal of green hydrogen-storage strategies carefully. In any event, having briefly discussed the 'green-storage' context and motivation for hydrogen storage in hydrates (especially the 'pure' case), together with some controversy over large-cage (and even small-cavity) occupancies giving rise to perhaps inflated H\({}_{2}\)-occupancy estimates [7; 8; 9; 10], the question remains to be answered about the fundamentals of intra-cage guest (H\({}_{2}\) and D\({}_{2}\)) phenomena; notably their structure, vibrations and'site-hopping' behaviour in the large 51264 sII-lattice cages. Although there has certainly been a number of high-quality studies on guest inter-cage dynamics and diffusivity throughout the sII lattice [16; 17; 18], as well as inter-cage H\({}_{2}\)-transfer energy barriers [11; 19; 20; 21], such intra-cage dynamics have been less studied. [PERSON] et al.