Arbitrary Force-Constant Changes in the Crystal Impurity Problem
  In a previous study of the dynamics of crystals with substitutional point defects we had obtained simple and exact expressions for the positions of the perturbed crystal modes and the intensities in them in the case in which both the host-host and the host-impurity force-constants were taken to be central and nearest neighbor. Such expressions required knowledge of only one pure crystal lattice Green's function, the one at the defect site itself. In this paper we extend our previous study to incorporate non-central force-constants as well. We find that the same simple expressions which we had previously obtained in the nearest neighbor central force-constant case also hold without any modification at all in the case of isotropic force-constant changes in crystals whose host-host force-constants are isotropic, and in the particular mixed case in which the fractional changes in the central and isotropic components of the force-constants are equal to each other. In the most general arbitrary nearest neighbor force-constant case we obtain a reasonably compact exact expression for the dynamics of the perturbed modes which only involves a total of two pure crystal lattice Green's functions. 
Philip D. Mannheim