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A tutorial review is given of the theory of crystals with large-amplitude, highly anharmonic vibrational motion. Applications are discussed to the lighter solid rare gases, particularly helium, as well as to the ferroelectric-lattice structural transformation. The single-particle and the collective pictures of the starting coordinate basis set are developed and compared. Formulas are presented for the phonon damping in the collective picture. A survey is outlined of Jastrow function treatments of short-range correlation due to hard-core interatomic repulsion in solid helium.
N. R. Werthamer (1969) studied this question.