We discuss the response of a crystal in the presence of a neutron distortion in the first-order self-consistent approximation and we stress the importance of the self-consistency condition. The resultant integral equation for the phonon energies is solved by direct matrix inversion instead of truncating a series expansion. Numerical results are presented for solid Ne and fcc ${\mathrm{He}}^4$ at 10.0 and 11.5 ${\mathrm{cm}}^3$/mole.

• Received 14 April 1970

DOI:https://doi.org/10.1103/PhysRevLett.24.1424