CONTRIBUTION OF 3-BODY OVERLPA FORCES TO DYNAMICAL MATRIX OF ALKALI HALIDES

The so‐called S2‐approximation in the Heitler‐London theory of ionic crystals as developed by Löwdin gives rise to a three‐body contribution in the cohesive energy. The present paper describes a method of deriving the coupling coefficients for the three‐body potential from expressions given by Lundqvist. These coupling coefficients involve certain lattice sums which can be evaluated in terms of incomplete gamma functions by a method due to Born and Bradburn. A new lattice dynamical model of ionic solids has been suggested which is obtained by injecting the three‐body coupling coefficients into the shell model of Woods et al. in a way that preserves symmetry in the wave vector space. Preliminary calculations show that this model can prove superior to other current models. Copyright © 1969 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim