Transferable Ion Force Fields in Water from a Simultaneous Optimization of Ion Solvation and Ion–Ion Interaction

Abstract

The poor performance of many existing nonpolarizable ion force fields is typically blamed on either the lack of explicit polarizability, the absence of charge transfer, or the use of unreduced Coulomb interactions. However, this analysis disregards the large and mostly unexplored parameter range offered by the Lennard-Jones potential. We use a global optimization procedure to develop water-model-transferable force fields for the ions K + , Na + , Cl – , and Br – in the complete parameter space of all Lennard-Jones interactions using standard mixing rules. No extra-thermodynamic assumption is necessary for the simultaneous optimization of the four ion pairs. After an optimization with respect to the experimental solvation free energy and activity, the force fields reproduce the concentration-dependent density, ionic conductivity, and dielectric constant with high accuracy. The force field is fully transferable between simple point charge/extended and transferable intermolecular potential water models. Our results show that a thermodynamically consistent force field for these ions needs only Lennard-Jones and standard Coulomb interactions.

Magnesium Force Fields for OPC Water with Accurate Solvation, Ion-Binding, and Water-Exchange Properties: Successful Transfer from SPC/E

“…4-site water models (TIP4P-D, followed by TIP4P-Ew, and TIP4P/2005) lead to larger deviations despite the fact that OPC is also a 4-site model (Table S4). This result has not been expected based on previous work which showed reasonable transferability of ion parameters within water models of the same complexity 1,2,67 .…”

Specific Ion Effects in Different Media: Current Status and Future Challenges

“…As further contributions, also the decisive role of local interactions in combination with charge transfer effects were discussed, ,,,,,− which directly point to modified electrostatics in terms of a so-called charge scaling concept for atomistic molecular dynamics (MD) simulations . Herewith, one can empirically introduce the missing polarizability of ions in MD simulations, such that the reduced charge of an ion can be written as with the dielectric constant ϵ r of the solution as consequence of polarization and charge transfer effects. , In addition to such static continuum approaches, also fully polarizable force fields in terms of Drude oscillators or bead–spring models for atomistic and coarse-grained MD simulations were introduced. − Thus, all of these approaches focus on fluctuating charge behavior which underlines the crucial role of ion–solvent interactions. In summary, one can conclude that the electronic polarizability in combination with charge transfer effects are important contributions for a more reliable description of specific ion effects in terms of the binding behavior, pairing effects, and ion distributions. − Moreover, it was also discussed that charges in solution introduce a locally varying dielectric permittivity which further affects the electrostatic interactions. , The corresponding higher order arrangement of the solvent molecules changes the local viscosity of the solution, such that also the dynamic behavior of the species is modified .…”

Molecular Simulations of Aqueous Electrolytes: Role of Explicit Inclusion of Charge Transfer into Force Fields

“…Thus, very recently, Loche et al using a fine-tuned nonpolarizable optimized FF simulated aqueous solutions of NaCl, KCl, NaBr, and KBr up to a concentration of 5 mol/kg. 16 Their new optimized FF reproduced the density, dielectric decrement, and ion conductivity variations of the solution over the entire concentration range that was studied. What about simulations of inhomogeneous systems, where inclusion of explicit polarization seems to be important?…”

“…After all, the pairwise potentials still contain a large number of parameters that can be fine-tuned. Thus, very recently, Loche et al using a fine-tuned nonpolarizable optimized FF simulated aqueous solutions of NaCl, KCl, NaBr, and KBr up to a concentration of 5 mol/kg . Their new optimized FF reproduced the density, dielectric decrement, and ion conductivity variations of the solution over the entire concentration range that was studied.…”