Assessing Many-Body Effects of Water Self-Ions. I:
OH<sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> Clusters
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Abstract
The
importance of many-body effects in the hydration of the hydroxide
ion (OH<sup>–</sup>) is investigated through a systematic analysis
of the many-body expansion of the interaction energy carried out at
the CCSD(T) level of theory, extrapolated to the complete basis set
limit, for the low-lying isomers of OH<sup>–</sup>(H<sub>2</sub>O)<sub><i>n</i></sub> clusters, with <i>n</i> = 1–5. This is accomplished by partitioning individual fragments
extracted from the whole clusters into “groups” that
are classified by both the number of OH<sup>–</sup> and water
molecules and the hydrogen bonding connectivity within each fragment.
With the aid of the absolutely localized molecular orbital energy
decomposition analysis (ALMO-EDA) method, this structure-based partitioning
is found to largely correlate with the character of different many-body
interactions, such as cooperative and anticooperative hydrogen bonding,
within each fragment. This analysis emphasizes the importance of a
many-body representation of inductive electrostatics and charge transfer
in modeling OH<sup>–</sup> hydration. Furthermore, the rapid
convergence of the many-body expansion of the interaction energy also
suggests a rigorous path for the development of analytical potential
energy functions capable of describing individual OH<sup>–</sup>–water many-body terms, with chemical accuracy. Finally, a
comparison between the reference CCSD(T) many-body interaction terms
with the corresponding values obtained with various exchange-correlation
functionals demonstrates that range-separated, dispersion-corrected,
hybrid functionals exhibit the highest accuracy, while GGA functionals,
with or without dispersion corrections, are inadequate to describe
OH<sup>–</sup>–water interactions