The nonadditive interactions in the Ar2HF and Ar2HCl clusters: An ab initio study.

The three-body effects in the Ar2HX (X=F, Cl) are studied by means of the supermolecular Mo\ller–Plesset perturbation theory in conjunction with the perturbation theory of intermolecular forces. In both systems the nonadditive interactions are large and repulsive around the equilibrium geometry. The in-plane bending potential of H–F in the Ar2HF cluster reveals a double minimum with the barrier of ca. 2–3 cm-1. The barrier is due to the three-body interactions. In Ar2HCl the analogous potential has a single minimum, and the three-body effects make it shallower. The three-body interaction energy is dissected into its components such as exchange, polarization, and dispersion. The anisotropy of the total nonadditvity in Ar2HF is dominated by polarization and exchange effects, and, consequently, it can be well reproduced at the self-consistent field level of theory. The overall nonadditivity in Ar2HCl is quite similar in magnitude, but it displays a different composition. The most anisotropic is polarization followed by dispersion and exchange effects. The dispersion effect is the largest in magnitude. The long-range part of the polarization nonadditivity is analyzed via the multipole approximation. The calculations indicate that the multipole expansion carried out to the inverse 12 power of R correctly describes the anisotropy of this effect. In Ar2HF the effect of intrasystem correlation on dispersion nonadditivity is also investigated via the supermolecular coupled cluster calculations. These calculations provide an estimate of 13.8 cm-1 for the total three-body effect in the equilibrium Ar2HF trimer. This value may still be underestimated due to the basis-set unsaturation of dispersion nonadditivity. [ABSTRACT FROM AUTHOR]