Rotational molecular motion in solid H2 and D2 under pressure

The rotational state of a H₂ or D₂ molecule in solid hydrogen or deuterium is calculated over the range of densities for which the molecular phases are expected to be stable. For para-H₂ and ortho-D₂ we predict sudden transitions as the pressure is increased from states where the molecules are rotating freely to states where they are librating about equilibrium orientations. These transitions are predicted to occur in the region of several hundred megabars of pressure. Although in principle observable, these first-order transitions are expected to lead to only small changes in the equation-of-state curves. These transitions from rotating to librating molecular motion are expected to be accompanied by changes in the crystal structures from hcp to fcc.Supported by NSF Grant No. 22553.