AB-INITIO STUDIES OF CYCLIC WATER CLUSTERS (H2O)(N), N=1-6 .2. ANALYSIS OF MANY-BODY INTERACTIONS

The magnitudes of the two- through six-body energy terms and their con tribution to the interaction energy of small ring water clusters (n=2- 6) are computed at the Hartree-Fock (HF) and, second-through fourth-or der many-body perturbation (MP2, MP4) levels of theory. The analysis i s performed at the minimum energy geometries reported earlier [J. Chem . Phys. 99, 8774 (1993)]. The correlation correction is found to accou nt for a 10%-20% increase in the individual two-body terms and a much larger increase of 75% for the three-body and 200% for the small four- body terms. The MP4 results are found to differ only slightly (<2%) fr om the corresponding MP2 results. We have found that three-body terms have a significant contribution as high as 30% to the interaction ener gy of the larger clusters and that four-body and higher order-terms ar e negligible. The total and incremental association energies for;the p rocesses n H2O-->(H2O)(n) and (H2O)(n-1) + H2O-->(H2O)(n), n=2-6 are a lso reported.