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-(H2O)(4) and formamide-(H2O)(3) clusters has been performed. The calculations were done at ab initio Hartree-Fock (HF) and Moller-Plesset (MP) levels. We split the nonadditive contributions to interaction energies 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 description of the bulk solvent effects we have employed the polarizable continuum model. Specific contributions are short ranged and involved interactions between individual molecules that in our case are described in the supermolecule approach. In the ammonium-(H2O)(4) system both contributions are important and it is necessary to employ a semicontinuum model. On the contrary, in the formamide-(H2O)(3) system the most important contribution is provided by the bulk solvent and a continuum model is satisfactory.