Lattice vibrations tunneling rotation

One ean distinguish three contributions to the probability density (i) the rotational dynamics (ii) the deformation of the methyl groups due to internal vibrations (iii) the lattice dynamics and/or disorder. Because the tunnelling occurs at a very low frequency, lattice dynamics and internal vibrations can be represented with time/space-averaged distributions. It is convenient to distinguish isotropic and anisotropic contributions. The former, as opposed to the latter, does not disturb the threefold symmetry. 

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