  Accurate predictions of the hydration free energy for anions typically have
been more challenging than for cations. Hydrogen bond donation to the anion in
hydrated clusters such as $\mathrm{F(H_2O)}_n{}^-$ can lead to delicate
structures. Consequently, the energy landscape contains many local minima, even
for small clusters, and these minima present a challenge for computational
optimization. Utilization of cluster experimental results for the free energies
of gas-phase clusters shows that, even though anharmonic effects are
interesting, they need not be troublesome magnitudes for careful applications
of quasi-chemical theory to ion hydration. Energy-optimized cluster structures
for anions can leave the central ion highly exposed and application of implicit
solvation models to these structures can incur more serious errors than for
metal cations. Utilizing cluster structures sampled from ab initio molecular
dynamics simulations substantially fixes those issues.
