Model polarizabilities and multipoles for ionic compounds. Alkali halides

Various dipole interaction models were compared with one another and with literature values for dissociation energies, multipole moments (u, Q, and Ω), and polarizabilities α for the alkali halide molecules. A displaced-shell (D-shell) model, which takes account of the overlap repulsion as in the shell model but also uses the effective electron shell displacements in calculating the electrostatic interactions, gives slightly better dissociation energies and as good or better dipole and quadrupole moments than the other models. All of the models considered here give mean polarizabilities in fairly good agreement with published SCF values; however, none gives consistently good polarizability anisotropies. It is found that both the overlap repulsion (as treated in the shell model) and the effective displacements of the electron shells (as treated in the D-shell model) have large effects on the calculated polarizability anisotropies relative to the DID model.