An Extended Breathing Shell Model for Alkali Halides and Its Test at Experimental Results

Group‐theoretical as well as quantum‐mechanical arguments require an extension of Schröder's breathing shell model (BSM) to include axial deformations. An extended model (EBSM) is presented and numerical results are given for KBr and KI. The dispersion curves do not differ appreciably from those of the BSM and the agreement with the experimental curves is equally good. In contrast, however, the eigenvectors, as tested indirectly via a calculation of the U‐center sideband, evidently are improved by the EBSM.