Ab initio molecular dynamics calculations of ion hydration free energies

TL;DR

This study uses ab initio molecular dynamics with thermodynamic integration to accurately compute hydration free energies of ions, achieving close agreement with experimental data and providing insights into redox reactions in water.

Contribution

First application of AIMD with thermodynamic integration to calculate ion hydration free energies with high accuracy, including transition metal redox reactions.

Findings

AIMD hydration free energies are within 4% of experimental values.

Sum of Li+/Cl- and Ag+/Cl- hydration energies closely match experiments.

Predictions suggest revisions for hydration energies of unstable radiolysis intermediates.

Abstract

We apply ab initio molecular dynamics (AIMD) methods in conjunction with the thermodynamic integration or "lambda-path" technique to compute the intrinsic hydration free energies of Li+, Cl-, and Ag+ ions. Using the Perdew-Burke-Ernzerhof functional, adapting methods developed for classical force field applications, and with consistent assumptions about surface potential (phi) contributions, we obtain absolute AIMD hydration free energies (Delta G(hyd)) within a few kcal/mol, or better than 4%, of Tissandier 's [J. Phys. Chem. A 102, 7787 (1998)] experimental values augmented with the SPC/E water model phi predictions. The sums of Li+/Cl- and Ag+/Cl- AIMD Delta G(hyd), which are not affected by surface potentials, are within 2.6% and 1.2 % of experimental values, respectively. We also report the free energy changes associated with the transition metal ion redox reaction Ag++Ni+-> Ag+Ni2+ in water. The predictions for this reaction suggest that existing estimates of Delta G(hyd) for unstable radiolysis intermediates such as Ni+ may need to be extensively revised.