Utility of chemical computations in predicting solution free energies of metal ions

Abstract

Here, we study quasi-chemical theory (QCT) for the free energies of divalent alkaline earth ions (Ba, Sr, Ca, Mg) in water, emphasizing that: (a) interactions between metal ions and proximal water molecules are as strong as traditional chemical effects; (b) QCT builds directly from accessible electronic structure calculations but rests on fully elaborated molecular statistical thermodynamics; (c) QCT offers choices of convenience in identifying coordination numbers for analysis. We investigate utilisation of direct QCT with inner-shell conditioning , alternative to the traditional conditioning motivated by a generalised van der Waals view. The alternative works well: deleterious non-Gaussian effects of van der Waals repulsive interactions are not serious, and the alternative conditioning improves the convenience of QCT calculations. Comparison between ab initio and force field molecular dynamics (AIMD and FFMD) with standard models suggests that FFMD likely exaggerates the anharmonicity in the thermal motion of inner-shell ion-water clusters. Together with the general encouraging support for the harmonic approximations implied by the conditioning, that observation helps explain the remarkable success of the cluster-based QCT solution free energies, which do not require assessment of all inner-shell occupancies by simulation.

Keywords:

• Quasi-chemical theory
• ion solvation
• ion channel blockers

Notes

No potential conflict of interest was reported by the authors.

Additional information

Funding

Sandia National Laboratories (SNL) is a multi-mission laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the USA Department of Energy’s National Nuclear Security Administration (Contract number DE-AC04-94AL8500). The financial support of Sandia’s LDRD programme, the Defense Threat Reduction Agency (DTRA) and the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the USA Department of Energy (Contract number DE- AC02-05CH11231, Sub-contract number 7060634) under the Advanced Batteries Materials Research (BMR) is greatly appreciated. The work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the USA DOE’s Office of Science by Los Alamos National Laboratory (Contract DE- AC52-06NA25396) and SNL.