Ab initio calculations of nonadditive effects in the trimers (H2O)2⋯XY, XY=N2, BF, CS

Optimal structures, interaction energies and harmonic vibrational frequencies of the (H2O)2⋯XY, XY=N2, BF, CS ternary complex have been determined from the supermolecular (SM) calculations with the aug-cc-pVDZ and the aug-cc-pVTZ basis sets. Energetic properties of the complex have been calculated at the MP4 level. We located three low-energy configurations corresponding to two isomeric H-bonded cyclic complexes and one linear structure. Nonadditive interactions play an important role for the (H2O)2⋯BF and (H2O)2⋯CS trimers. The contribution of the three-body term represents as much as 17% and 13% of the total MP4 interaction energy. Partitioning of the three-body energy was performed in terms of the intermolecular perturbation theory. The nonadditivity originates mainly from the induction effect. The calculations of the vibrational frequencies and infrared intensities for these complexes are presented to facilitate the frequency assignments of future experimental spectra.