ABOUT THE NATURE OF INTERMOLECULAR 3-BODY FORCES IONIC SYSTEMS - THE CASE OF PROTONATED HYDRATES

The three-body intermolecular energy is computed for the water/oxonium ion/water system from ab initio self-consistent-field (SCF) calculati ons and 170 geometrical configurations. It is decomposed into two part s: Delta E(nonadd)(1st) is obtained at the first iteration of the calc ulation and Delta E(nonadd)(deloc) is due to the following iterations till convergency The nonadditive three-body energy, E(nonadd)(ind), is computed from the perturbation theory. It is shown that all these ene rgy components can be either attractive or repulsive according to the geometry. E(nonadd)(deloc) is often seriously different of E(nonadd)(i nd), a nonadditive ''induction-exchange'' term being sometimes as larg e as E(nonadd)(ind). In most cases, this ''induction-exchange'' term i s much larger than E(nonadd)(1st), with the same sign. This Suggests t hat these two terms can be approximated by a same expression, but beca use of the sign, more than one exponential term should be used. An app roximation of E(nonadd)(ind) is discussed with a particular attention to the range of the validity according to the intermolecular distances .