  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.
