The many-body expansion for aqueous systems revisited
Advanced Computing, Mathematics and Data Division, Pacific Northwest National Laboratory, 902 Battelle Boulevard, P.O. Box 999, MS K1-83, WA, 99352, USA Department of Chemistry, University of Washington, Seattle, WA 98195, USA We revisit the Many-Body Expansion (MBE) for water-water interactions by examining the effects of the basis set, including those resulting from the Basis Set Superposition Error (BSSE) correction, on the various terms for selected sizes of water clusters up to n = 21. The analysis is performed at the second order Møller-Plesset (MP2) perturbation theory with the family of augmented correlation consistent basis sets up to five zeta\ quality for the (H2O)n, n = 7, 10, 13, 16 and 21 clusters, for which we report either the complete MBE (n = 7, 10) or the one through the 6-body (n = 13) and the 5-body terms (n = 16, 21). Our results suggest that any sizeable contributions to the total cluster binding energy arising from the 5-body and larger terms are solely an artifact of the finite basis set. Indeed, all terms above the 4-body converge to practically zero at the Complete Basis Set (CBS) limit and this finding is accurately reproduced even with the smaller basis set of the series (aug-cc-pVDZ) once the BSSE correction is considered. The same level of theory (MP2/aug-cc-pVDZ, BSSE-corrected) also accurately reproduces the magnitude of the 3- and 4-body terms, for which we also find that the contributions of electron correlation are quite small. Our results unquestionably demonstrate that the MBE for water-water interactions vanishes monotonically with basis set size and can be safely truncated at the 4-body term once BSSE corrections are considered. We expect these findings to have important consequences in the pursuit of accurate many-body molecular dynamics simulations for aqueous systems.
* This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. Pacific Northwest National Laboratory (PNNL) is a multi program national laboratory operated for DOE by Battelle