Abstract
We report the first fully coupled quantum 6D bound-state calculations of the vibration-translation-rotation (VTR) eigenstates of a fexible H$_2$, HD, and D$_2$molecule confined in the small cage of the sII clathrate hydrate embedded in larger hydrate domains, treated as rigid. These calculations extend to the first excited (v = 1) vibrational state of H$_2$ and the two isotopologues, and allow direct computationof its vibrational frequency shift. For the caged H$_2$, the fundamental translational excitations, the rotational $j = 0 \rightarrow 1$ transitions, and the frequency shifts of the stretch fundamental from these quantum 6D calculations are in excellent agreement with the corresponding results of the recent quantum 5D (rigid H$_2$) treatment [A. Powers et al., J. Chem. Phys. 148, 144304 (2018)].