Ab initio molecular dynamics calculations of ion hydration free energies

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 (DeltaG(hyd)) within a few kcal/mol, or better than 4%, of Tissandier et al.'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 DeltaG(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+Ni(2+) in water. The predictions for this reaction suggest that existing estimates of DeltaG(hyd) for unstable radiolysis intermediates such as Ni(+) may need to be extensively revised.

Response to “Comment on ‘Ab initio molecular dynamics calculation of ion hydration free energies’ [J. Chem. Phys. 133, 047103 (2010)]”

“…7 With DFT methods, we have so far unambiguously calculated q . 1 2 Even better agreement is obtained when comparing neutral ion pairs where ambiguities in cancel. To reiterate, no parameters in our model are adjusted to achieve a fit to experiment.…”

“…To evaluate the quality of these predictions, we made a proper apples-to-apples comparison to tabulated data from experimental work. 2,3 The comment by Chen and Chen 4 reflects confusion about our choice of observable used to achieve a consistent comparison of predicted 1 and tabulated 2,3 ⌬G hyd , perhaps incorrectly implies that we adjusted free parameters to achieve a fit to experimental data, and overstates the utility of the Born hydration formula for ion hydration. Some of their apparent disagreement with our work appears more philosophical than technical in nature, but we welcome the opportunity to clarify our results in light of recent developments in this field.…”

“…In our recent work, 1 we applied ab initio molecular dynamics ͑AIMD͒ simulations based on density functional theory ͑DFT͒ to predict the hydration free energy ͑⌬G hyd ͒ of several simple ions for the first time. To evaluate the quality of these predictions, we made a proper apples-to-apples comparison to tabulated data from experimental work.…”

First principles view on chemical compound space: Gaining rigorous atomistic control of molecular properties

“…[42] Sulpizi and Sprik [43] rigorously explored the need for fractional nuclear charge calculations to predict pK a 's of various organic and inorganic acids and bases. In the case of free energy differences, fractional charges can also be avoided all together within a simple alternative and elegant interpolation scheme put forth by Alfè et al [44] : atomic forces are evaluated at both endpoints (k 2 0; 1), and k dependent molecular dynamics trajectories are generated for atoms being propagated according to a linear combination of these forces using instantaneous k values as weights.…”

Molten salt eutectics from atomistic simulations

“…Furthermore, in the absence of accurate interatomic potentials it would be straightforward to use AIMD within the presented scheme. Alchemical changes within AIMD can be performed in a manner analogous to that described here [32,33]. The corresponding anticipated increase in computational cost could be mitigated by reducing the necessary number of mole fractions or through coarser integration of Eq.…”