Lattice dynamic theory of compressed rare-gases crystals in the deformed   atom model
  Lattice dynamics of rare-gas crystals is built on the base of adiabatic approximation when the deformation of electron shells of atoms of dipole and quadrupole types depending on nucleus shift and simultaneously arising Van-der-Vaals forces. The dipole forces are the most long-range ones. The obtained oscillation equations are studied in long-wave approximation. The role of three-body interaction and quadrupole deformation in the violation of Cauchy relation is discussed. Birch elastic moduli calculated for Xe and deviations from Cauchy relation are in good agreement with the experiment in a wide pressure range. 
V. N. Varyukhin, E. P.Troitskaya, V. V.Chabanenko, I. V. Zhikharev, E.   E. Gorbenko, E. A. Pilipenko
cond-mat.other