SPECIFIC AND BULK SOLVENT NONADDITIVE CONTRIBUTIONS TO THE IN-SOLUTION BINDING-ENERGY OF AMMONIUM-WATER CLUSTERS

A study of the solvent effect on the interaction energy of ammonium-(H 2O)(4) and formamide-(H2O)(3) clusters has been performed. The calcula tions were done at ab initio Hartree-Fock (HF) and Moller-Plesset (MP) levels. We split the nonadditive contributions to interaction energie s in solution into specific and bulk solvent terms. Bulk solvent terms are due to polarization effects, which are statistical in nature and are originated by a collective response of the solvent. In the descrip tion of the bulk solvent effects we have employed the polarizable cont inuum model. Specific contributions are short ranged and involved inte ractions between individual molecules that in our case are described i n the supermolecule approach. In the ammonium-(H2O)(4) system both con tributions are important and it is necessary to employ a semicontinuum model. On the contrary, in the formamide-(H2O)(3) system the most imp ortant contribution is provided by the bulk solvent and a continuum mo del is satisfactory.