Correlation and basis set superposition effects on three body ion-water-water interactions

Ab initio SCF and second order Moller-Plesset electronic calculations using a point charge (PC) model as well as standard basis sets for the ions (Li+, Mg++) have been performed for several different water-water geometries and distances. The PC model yields acceptable ion-water potential curves. The ab initio curves calculated with and without Basis Set Superposition Error and electron correlation corrections as well as via a classical model are compared for various water dimer orientations. Certain geometries (particularly those appropriate to the interaction of hydrated ions) gave large Basis Set Superposition Errors and large correlation effects (˜20%) even with the large basis set (Clementi – (11,7,1/6,1) → [4,3,1/2,1]) used. For attractive potentials this lead to some cancellation but it is compounded for repulsive curves. The classical model used was able to predict qualitative trends in most cases indicating the nonadditivity arises primarily from an inductive effect; however, geometries with strong hydrogen-hydrogen repulsions show an attractive three body potential at short distances which cannot be understood in terms of this classic model.