Accurate Prediction of the Hydration Free Energies of 20 Salts through Adaptive Force Matching and the Proper Comparison with Experimental References

by Li, Jicun; Wang, Feng

Simple pairwise potentials for five alkali ions and four halide ions were developed by only fitting to ab initio MP2 forces with the adaptive force matching (AFM) method. Without fitting to any experimental information, the AFM models predict the hydration free energies of all 10 fluoride and chloride salts formed by these ions within 1.5% of experimental references. The predicted hydration free energies for the 10 bromide and iodide salts are within 5-6% of experimental references with the larger error likely due to the neglect of explicit treatment of polarization and charge transfer. An inconsistency in the treatment of the gas phase entropy term between experimental and theoretical approaches is discussed. A new simplified hydration free energy for the ions is reported for use as a more appropriate experimental reference for further theoretical studies. The simulations show different dipole alignments for the hydration waters of cations and anions. While hydration waters of small cations tend to align their molecular dipole toward the ion, the dipole of one of the water OH bonds is aligned with the field of an anion.

Journal
Journal of Physical Chemistry B
Volume
121
Issue
27
Year
2017
Start Page
6637
ISBN/ISSN
1520-5207; 1520-6106
PMID
28621540
DOI
10.1021/acs.jpcb.7b04618