Interfacial ion solvation: Obtaining the thermodynamic limit from molecular simulations

TL;DR

This paper develops a method to correct finite size effects in molecular simulations of ion solvation near interfaces, enabling accurate extrapolation to the thermodynamic limit using dielectric continuum theory.

Contribution

It introduces a new finite size correction approach for interfacial ion solvation simulations based on dielectric continuum theory, improving accuracy over previous bulk solution methods.

Findings

The correction accurately predicts solvation free energy variation with system size and shape.

It accounts for solvent charge asymmetry, improving results for aqueous systems.

The method enables reliable extrapolation to the thermodynamic limit in interfacial simulations.

Abstract

Inferring properties of macroscopic solutions from molecular simulations is complicated by the limited size of systems that can be feasibly examined with a computer. When long-ranged electrostatic interactions are involved, the resulting finite size effects can be substantial and may attenuate very slowly with increasing system size, as shown by previous work on dilute ions in bulk aqueous solution. Here we examine corrections for such effects, with an emphasis on solvation near interfaces. Our central assumption follows the perspective of H\"{u}nenberger and McCammon [J. Chem. Phys. 110, 1856 (1999)]: Long-wavelength solvent response underlying finite size effects should be well described by reduced models like dielectric continuum theory, whose size dependence can be calculated straightforwardly. Applied to an ion in a periodic slab of liquid coexisting with vapor, this approach yields a finite size correction for solvation free energies that differs in important ways from results previously derived for bulk solution. For a model polar solvent, we show that this new correction quantitatively accounts for the variation of solvation free energy with volume and aspect ratio of the simulation cell. Correcting periodic slab results for an aqueous system requires an additional accounting for the solvent's intrinsic charge asymmetry, which shifts electric potentials in a size-dependent manner. The accuracy of these finite size corrections establishes a simple method for a posteriori extrapolation to the thermodynamic limit, and also underscores the realism of dielectric continuum theory down to the nanometer scale.