Assessment of Multicoefficient Correlation Methods, Second-Order Møller−Plesset Perturbation Theory, and Density Functional Theory for H₃O⁺⁽H₂O)_n (n = 1−5) and OH^-(H₂O)_n (n = 1−4)

We have assessed the ability of 52 methods including 15 multicoefficient correlation methods (MCCMs), two complete basis set (CBS) methods, second-order Møller−Plesset perturbation theory (MP2) with 5 basis sets, the popular B3LYP hybrid functional with 6 basis sets, and 24 combinations of local density functional and basis set to accurately reproduce reaction energies obtained at the Weizmann-1 level of theory for hydronium, hydroxide, and pure water clusters. The three best methods overall are BMC-CCSD, G3SX(MP3), and M06-L/aug-cc-pVTZ. If only microsolvated ion data is included, M06-L/aug-cc-pVTZ is the best method; it has errors only half as large as the other density functionals. The deviations between the three best performing methods are larger for the larger hydronium- and hydroxide-containing clusters, despite a decrease in the average reaction energy, making it impossible to determine which of the three methods is overall the best, so they might be ranked in order of increasing cost, with BMC−CCSD least expensive, followed by M06-L/aug-cc-pVTZ. However, the cost for M06-L will increase more slowly as cluster size increases. This study shows that the M06-L functional is very promising for condensed-phase simulations of the transport of hydronium and hydroxide ions in aqueous solution.