Simulating Monovalent and Divalent Ions in Aqueous Solution Using a Drude Polarizable Force Field

Abstract

An accurate representation of ion solvation in aqueous solution is critical for meaningful computer simulations of a broad range of physical and biological processes. Polarizable models based on classical Drude oscillators are introduced and parametrized for a large set of monoatomic ions including cations of the alkali metals (Li(+), Na(+), K(+), Rb(+) and Cs(+)) and alkaline earth elements (Mg(2+), Ca(2+), Sr(2+) and Ba(2+)) along with Zn(2+) and halide anions (F(-), Cl(-), Br(-) and I(-)). The models are parameterized, in conjunction with the polarizable SWM4-NDP water model [Lamoureux et al., Chem. Phys. Lett. 418, 245 (2006)], to be consistent with a wide assortment of experimentally measured aqueous bulk thermodynamic properties and the energetics of small ion-water clusters. Structural and dynamic properties of the resulting ion models in aqueous solutions at infinite dilution are presented.

Figure 1

SWM4-NDP water molecule with a point charge of +2e placed 2X, X and X/2 defined by Eq. 4 away from the oxygen atom.

Figure 2

Binding energies of the divalent monohydrates as a function of the distance between the ion and the oxygen atoms: Solid line: ab initio results; Dashed line: Drude. See Table 2 caption for the theory and basis set used to obtain the reference data.

Figure 3

Total dipole moments of the divalent monohydrates as a function of the distance between the ion and the oxygen atoms. The ions are located at the origin to compute the dipole moment. See Figure 2 for details.

Figure 4

Hydration free energies of salts with monovalent cations. Tissandier; Klots; Marcus; Noyes; Schmid; Randles; Gomer.

Figure 5

Hydration free energies of salts with divalent cations. Schmid:; Marcus:; Gomer:; Noyes:; and this work: see the caption of Table 3 for details; The entries for MgF₂ and SrF₂ in this work were derived based on hydration free energy differences between F⁻ and Cl⁻ and the total hydration free energies of salts MgCl₂ and SrCl₂.

Figure 6

Radial hydration structure for the alkali cations. Radial distribution functions g(R_Ion−O) functions are shown in solid line and the coordination numbers N(R_Ion−O) are shown in dashed line.

Figure 7

Radial hydration structure for the halide anions. Radial distribution functions g(R_Ion−O) functions are shown in solid line and the coordination numbers N(R_Ion−O) are shown in dashed line.

Figure 8

Radial hydration structure for the divalent cations. Radial distribution functions g(R_Ion−O) functions are shown in solid line and the coordination numbers N(R_Ion−O) are shown in dashed line.