Molecular dynamics simulations based on pseudopotentials are used to characterize the difference between impurity rotations in classical versus quantum solids. The method is first applied to the pure solids and demonstrated to faithfully reproduce static and dynamical properties, in the form of pair distributions and phonon density of states of solid H 2 ͑D 2 ͒. Then the rotations of molecular oxygen in the ground X( 3 ⌺ g Ϫ ) and electronically excited state AЈ( 3 ⌬ u ) is investigated. Where

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... he substitutional impurity is small, O 2 (X), in the classical solid, the cavity remains nearly spherical and the molecule undergoes rotation-translation coupled motion. In contrast, in the quantum solid, the lattice locally distorts around the impurity and forces librations with occasional reorientational hops as rotation-distortion coupled motion. These effects are amplified in the excited O 2 (AЈ) state, in which due to the larger molecular bond length, the angular anisotropy of the guest-host interaction is larger. Now, in the classical solid a small cage distortion forces the molecule into large amplitude librations. The molecule, however, reorients occasionally, when the lattice fluctuations lead to a nearly spherical cage geometry. In the quantum host, O 2 (AЈ) becomes a strict librator, due to a large and permanent deformation of the soft cage. The results are used to rationalize experimental observations.