The anharmonicity in solid crystals due to the nonlinearity
of interatomic forces with respect to atomic displacements, is
the origin of many important thermodynamic properties of
crystals such as the thermal expansion or the deviation of the
lattice specific heat at high temperatures with respect to Debye
contribution. Moreover, the anharmonic nature of lattice
vibrations in crystals plays an essential role in determining
the lattice dynamics in the vicinity of structural phase transitions..
