On three body forces in rare gas crystals
The effect of three body v.d. Waals forces on the elastic constants of solid argon, krypton and xenon is calculated. It is pointed out that three body forces and zero-point motion have the opposite effect on the violation of the Cauchy relationc12=c44. Due to three body forces the quantitityδ=c44−c12/c12 is changed from about +4 to −4% for argon, while in the case of xenonδ is shifted from + 1 to −9%.