An accurate and efficient computational protocol for obtaining the complete basis set limits of the binding energies of water clusters at the MP2 and CCSD(T) levels of theory: Application to (H2O)m, m=2-6, 8, 11, 16, and 17

We report MP2 and Coupled Cluster Singles, Doubles, and perturbative Triples [CCSD(T)] binding energies with basis sets up to pentuple zeta quality for the (H2O)(m=2-6,8) water clusters. Our best CCSD(T)/Complete Basis Set (CBS) estimates are -4.99 +/- 0.04 kcal/mol (dimer), -15.8 +/- 0.1 kcal/mol (trimer), -27.4 +/- 0.1 kcal/mol (tetramer), -35.9 +/- 0.3 kcal/mol (pentamer), -46.2 +/- 0.3 kcal/mol (prism hexamer), -45.9 +/- 0.3 kcal/mol (cage hexamer), -45.4 +/- 0.3 kcal/mol (book hexamer), -44.3 +/- 0.3 kcal/mol (ring hexamer), -73.0 +/- 0.5 kcal/mol (D-2d octamer), and -72.9 +/- 0.5 kcal/mol (S-4 octamer). We have found that the percentage of both the uncorrected (D-e) and basis set superposition error-corrected (D-e(CP)) binding energies recovered with respect to the CBS limit falls into a narrow range on either sides of the CBS limit for each basis set for all clusters. In addition, this range decreases upon increasing the basis set. Relatively accurate estimates (within <0.5%) of the CBS limits can be obtained when using the 2/3, 1/3 (for the AVDZ set) or the 1/2, 1/2 (for the AVTZ, AVQZ, and AV5Z sets) mixing ratio between D-e and D-e(CP). These mixing rations are determined via a least-mean-squares approach from a dataset that encompasses clusters of various sizes. Based on those findings, we propose an accurate and efficient computational protocol that can be presently used to estimate accurate binding energies of water clusters containing up to 30 molecules (for CCSD(T)) and up to 100 molecules (for MP2). (C) 2015 AIP Publishing LLC.