A quantum chemical molecular dynamics repository of solvated ions

Abstract

The importance of ion-solvent interactions in predicting specific ion effects in contexts ranging from viral activity through to electrolyte viscosity cannot be underestimated. Moreover, investigations of specific ion effects in nonaqueous systems, highly relevant to battery technologies, biochemical systems and colloid science, are severely limited by data deficiency. Here, we report IonSolvR - a collection of more than 3,000 distinct nanosecond-scale ab initio molecular dynamics simulations of ions in aqueous and non-aqueous solvent environments at varying effective concentrations. Density functional tight binding (DFTB) is used to detail the solvation structure of up to 55 solutes in 28 different protic and aprotic solvents. DFTB is a fast quantum chemical method, and as such enables us to bridge the gap between efficient computational scaling and maintaining accuracy, while using an internally-consistent simulation technique. We validate the database against experimental data and provide guidance for accessing individual IonSolvR records.

Fig. 1

Structure of bulk water obtained from a 1249 × 1249 × 1249 pm³ (/65 water molecule) PBC unit cell (ρ = 0.99707 g·cm^-3) using DFTB3-D3(BJ)/3ob-3-1 as a function of (a,c) timestep (fs) and (b,d) NHC coupling strength. Note that supplied data in IonSolvR use a consistent timestep of 1 fs and NHC coupling strength of 1000 cm⁻¹. Structure is gauged here via the intermolecular (a,b) O-O and (c,d) O-H radial distribution functions. Experimental O-O RDFs from Skinner et al. and O-H RDFs from Soper et al.^,. Sampling is performed after a 20 ps equilibration period.

Fig. 2

Gibbs free energies of ion hydration calculated using Eq. (1) using IonSolvR trajectories with 64 water molecules, compared to experimental values. X-axis error bars indicate the standard error in the simulated $\mathrm{\Delta }{\mathrm{G}}_{\mathrm{hyd}}\left(\mathrm{X}\right)$ value. Note that the line of best fit equation here accounts for the effective concentration dependence incurred by using 64 water molecule trajectories.

Fig. 3

The influence of effective concentration on ion solvation in (a,b) bulk water (simulation parameters as in Fig. 1) and (c,d) aqueous NaCl solution, and solvated Na⁺ and Cl⁻ ions (based on the number of water molecules in the PBC unit cell and the experimental density of 0.997 g∙cm⁻³). Structure is gauged here via the intermolecular (a) O-O, (b) O-H, (c) Na-O and (d) Cl-H radial distribution functions. Experimental data from Skinner et al., Soper et al.^, and Mancinelli et al. at an effective ion:water concentration of 1:83. Sampling is performed after a 20 ps equilibration period.