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

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.