Ab initio study of nonadditive interactions in the Ar2HF and Ar2HCl clusters. II. Analysis of exchange and induction effects

The nonadditive effects are studied in Ar2HX (X=F, Cl) clusters using the supermolecular Mo/ller–Plesset perturbation theory in conjunction with the perturbation theory of intermolecular forces. The range of geometrical variations include the rotation of HX within the plane of the cluster and perpendicular to it. The three-body effect is rigorously dissected into the three fundamental nonadditive components: exchange, induction, and dispersion. The exchange nonadditivity is further divided into two terms: ES3, the electrostatic interaction between the HX and Ar2 subunits due to the appearance of exchange-induced quadrupole moment on Ar2 [Cooper and Hutson, J. Chem. Phys. 98, 5337 (1993)], and its exchange counterpart X3. ES3 is physically interpreted as originating from the interatomic electron exchanges within the Ar2 part. The X3 term originates from triple exchanges among all three monomers. The induction nonadditivity is dominated by the third-order effect describing the interaction of moments induced on the Ar atoms by the field of HX. This effect is faithfully represented by the multipole approximation.