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Hydrated anions: From clusters to bulk solution with quasi-chemical theory

The interactions of hydrated ions with solution and interface partners are strong on a chemical energy scale. Here, we test the foremost \textit{ab initio theory} for evaluation of hydration free energies of ions, namely, \textit{quasi-chemical theory} (QCT). We focus on halide anions, but also the hydroxide anion, since they have been outstanding challenges for all theories. QCT is built by identification of inner-shell clusters, separate treatment of those clusters, then integration of those results into the broader-scale solution environment. We exploit a close comparison with mass-spectrometric measurements of ion-hydration equilibria. That theory-experiment comparison is excellent with moderate computational effort here. This agreement reinforces both theory and experiment, and provides a numerically accurate inner-shell contribution to QCT. The inner-shell complexes involving heavier halides display strikingly asymmetric hydration clusters. QCT provides a favorable setting for exploitation of the polarizable continuum model (PCM) when the inner-shell material shields the ion from the outer solution environment. For the asymmetrically hydrated, and less effectively shielded, heavier halide ions, we investigate an inverse procedure in which the inner-shell structures are sampled from readily available AIMD calculations on the bulk solutions. This inverse procedure is a remarkable improvement and our final results are in close agreement with a standard tabulation of hydration free energies. Comparison of anion hydration cluster structures with bulk solutions from AIMD simulations emphasize slight differences: the asymmetries of bulk solution inner-shell structures are moderated, but still present; and inner shells fill to slightly higher average coordination numbers in bulk solution than in clusters.