Ab initio studies of cyclic water clusters (H2O)n, n=1–6. II. Analysis of many-body interactions

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Abstract
The magnitudes of the two- through six-body energy terms and their contribution to the interaction energy of small ring water clusters (n=2–6) are computed at the Hartree–Fock (HF) and second- through fourth-order many-body perturbation (MP2, MP4) levels of theory. The analysis is performed at the minimum energy geometries reported earlier [J. Chem. Phys. 99, 8774 (1993)]. The correlation correction is found to account 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%) from the corresponding MP2 results. We have found that three-body terms have a significant contribution as high as 30% to the interaction energy of the larger clusters and that four-body and higher order terms are negligible. The total and incremental association energies for the processes n H2O→(H2O)n and (H2O)n−1+H2O→(H2O)n, n=2–6 are also reported.