Ab initio calculations of nonadditive effects in the trimers ðH 2 OÞ 2  XY; XY ¼ N 2 , BF, CS Michal F. Rode 1 , Joanna Sadlej * Department of Chemistry, University of Warsaw, Pasteur 1 Str., 02-093 Warsaw, Poland Received 12 March 2002; in final form 8 April 2002 Abstract Optimal structures, interaction energies and harmonic vibrational frequencies of the ðH 2 OÞ 2  XY, XY ¼ N 2 , 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. Non- additive interactions play an important role for the ðH 2 OÞ 2  BF and ðH 2 OÞ 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 pre- sented to facilitate the frequency assignments of future experimental spectra. Ó 2002 Published by Elsevier Science B.V. 1. Introduction Clusters of H 2 O with gaseous atoms and mol- ecules received considerable attention, both from experimentalists and from theorists [1–3]. The long term goal of the investigation of such clusters is to gain insight into interactions of small molecules with water in condensed phase systems. By studying the small clusters one can gain consider- able insight into exact information on nonaddi- tivity. Nonadditive interactions have been the subject of intensive investigations since the work of Axil- rod and Teller [4]. The water trimer is perhaps the most frequently studied of all (for a relatively re- cent review see [2,3]). It is frequently assumed that the interactions are pairwise additive, i.e., that the potential energy can be approximated as the sum of interaction between all pairs of constituent molecules in modeling condensed phase systems bound by hydrogen bonding and van der Waals interactions. However deviations from this ap- proximation are often substantial and have been a focus of intensive studies during the last decade [2,6]. It has been argued that the many body effects in water cannot be neglected in computer simula- tion of liquid water. The nonadditive effects in the 31 May 2002 Chemical Physics Letters 358 (2002) 237–249 www.elsevier.com/locate/cplett * Corresponding author. E-mail address: sadlej@chem.uw.edu.pl (J. Sadlej). 1 Also corresponding author. 0009-2614/02/$ - see front matter Ó 2002 Published by Elsevier Science B.V. PII:S0009-2614(02)00587-0