A PAIRWISE AND 2 MANY-BODY MODELS FOR WATER - INFLUENCE OF NONPAIRWISE EFFECTS UPON THE STABILITY AND GEOMETRY OF (H2O)(N) CYCLIC (N=3-6) AND CAGE-LIKE (N=6-20) CLUSTERS

Three flexible models (PW, CMP and TCPE) are presented to study intera ctions occurring in water clusters. The total interaction energy is de composed into five terms; repulsive, charge-charge, intramolecular rel axation, polarization, and hydrogen bonding. The parameters of the fir st three terms are the same for all of the three models. The polarizat ion term is taken as a classical molecular many-body polarization pote ntial for the CMP and TCPE models, while nonpairwise effects are omitt ed in the PW model. As nonpairwise effects occurring in water clusters greater than the dimer have a topological origin, such effects are in troduced in the hydrogen bonding term of the TCPE model. Parameters of the three models are derived from ab initio calculations at the MP2/6 -311+G(2df,2p) level on three water dimer structures and the cyclic wa ter trimer. Application of the three models to water clusters (H2O)(n) from n=3 to 20 exhibits that the nonpairwise effects in such clusters represent about 20% of their total binding energy and that the two no npairwise effects considered suffice to describe most of the many-body effects. In particular, the many-body polarization term is mostly res ponsible for interoxygen distance contractions (relative to the dimer) in clusters greater than n=3, In cyclic clusters, this term is respon sible for 40%-70% of the total nonpairwise effect energy, while in cag elike clusters, for about 50%. (C) 1997 American Institute of Physics.