Force fields for divalent cations based on single-ion and ion-pair properties

Abstract

We develop force field parameters for the divalent cations Mg(2+), Ca(2+), Sr(2+), and Ba(2+) for molecular dynamics simulations with the simple point charge-extended (SPC/E) water model. We follow an approach introduced recently for the optimization of monovalent ions, based on the simultaneous optimization of single-ion and ion-pair properties. We consider the solvation free energy of the divalent cations as the relevant single-ion property. As a probe for ion-pair properties we compute the activity derivatives of the salt solutions. The optimization of the ionic force fields is done in two consecutive steps. First, the cation solvation free energy is determined as a function of the Lennard-Jones (LJ) parameters. The peak in the ion-water radial distribution function (RDF) is used as a check of the structural properties of the ions. Second, the activity derivatives of the electrolytes MgY(2), CaY(2), BaY(2), SrY(2) are determined through Kirkwood-Buff solution theory, where Y = Cl(-), Br(-), I(-). The activity derivatives are determined for the restricted set of LJ parameters which reproduce the exact solvation free energy of the divalent cations. The optimal ion parameters are those that match the experimental activity data and therefore simultaneously reproduce single-ion and ion-pair thermodynamic properties. For Ca(2+), Ba(2+), and Sr(2+) such LJ parameters exist. On the other hand, for Mg(2+) the experimental activity derivatives can only be reproduced if we generalize the combination rule for the anion-cation LJ interaction and rescale the effective cation-anion LJ radius, which is a modification that leaves the cation solvation free energy invariant. The divalent cation force fields are transferable within acceptable accuracy, meaning the same cation force field is valid for all halide ions Cl(-), Br(-), I(-) tested in this study.

Pairwise-additive force fields for selected aqueous monovalent ions from adaptive force matching

“…53,54 The leading term of the correction is from the interaction between self-images in the reciprocal space in the Ewald method. For a single ion in the periodic box, Ewald will give a non-zero self energy.…”

Molecular Dynamics Simulations of Cetyltrimethylammonium Bromide (CTAB) Micelles and their Interactions with a Gold Surface in Aqueous Solution

“…In terms of ion-water interaction for the three bromide models, the measurement of the position of the first maximum radial distribution function of water around the ion (r max,1 ) and hydration number in the first hydration shell around (n h,O ), under similar conditions, is respectively, 3.4 Å and 6.5 for B H , 34 3.3 Å and 6.0 for B N , 35 and 3.5 Å and 7.6 for B R . 36 In this perspective, the bromide B N , with a relatively small hydration layer, should display a stronger interaction with the CTA + polar group.…”

“…The chloride Lennard-Jones atomic parameter (LJAP) was taken from the GROMOS parameter set gromos54a7. 32 The bromide LJAPs adopted for evaluation were those proposed by Hasse and co-workers 34 (B H ), Netz and co-workers 35 (B N ), and Reiser et al 36 (B R ). Finally, the LJAP employed for gold was that developed by Heinz et al 37 and was already used in previous studies.…”

Single-ion hydration thermodynamics from clusters to bulk solutions: Recent insights from molecular modeling

“…The largest group of the empirical models conforms to the same type of ion-water interactions, and consist of the Lennard-Jones (LJ) potential acting between the ion and water oxygen, complemented by point charge (PC) interactions between the ion and water oxygen and hydrogens [25,26,66,83,91,97,[153][154][155][156][157][158][159][160]. We will denote this force field form as LJPC.…”

“…Almost invariably, these empirical models were optimized to reproduce only a single thermodynamic function (either DG aq [25,26,97,154], or DH aq [155]), which obscured their inability to reproduce the full set of common thermodynamic propertiesGibbs free energy (G), enthalpy (H) and entropy (S) of hydration. This fact has only become apparent when Horinek et al [25] attempted to develop their simple non-polarizable model of the [29].…”