The Cu and Ag ion displacements in gamma-CuX (X = Cl, Br, I) and gamma
-AgI are calculated by using the transverse effective charge based on
a lattice-distortion model. These cation displacements are compared wi
th the root-mean-square displacements of cations obtained from the anh
armonic model for CuX and AgI. It is found that the lattice-distortion
model is consistent with the anharmonic model.