The combination of supermolecular Moslash;llerndash;Plesset treatment with the perturbation theory of intermolecular forces is applied in the analysis of the potentialhyphen;energy surface of Arndash;H2O. The surface is very isotropic with the lowest barrier for rotation of sim;35 cmminus;1above the absolute minimum. The lower bound forDeis found to be 108 cmminus;1and the complex reveals a very floppy structure, with Ar moving freely from the Hhyphen;bridged structure to the coplanar and almost perpendicular arrangement of the C2ndash;water axis and the Arndash;O axis, lsquo;lsquo;Thyphen;shapedrsquo;rsquo; structure. This motion is almost isoenergetic (energy change of less than 2 cmminus;1). The Hhyphen;bridged structure is favored by the attractive induction and dispersion anisotropies; the Thyphen;shaped structure is favored by repulsive exchange anisotropy. The nonadditive effect in the Ar2ndash;H2O cluster was also calculated. Implications of our results on the present models of hydrophobic interactions are also discussed.
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