Image Charge Solvation Model (ICSM) with Improved Boundary Conditions

Abstract

In previous work the Image Charge Solvation Model (ICSM) [1] was introduced as an alternative way to simulate bulk properties of solutions. The ICSM consists of a spherical cavity of explicit solvent embedded in a continuum dielectric medium, and solves the electrostatic interactions accurately and more efficiently than the Particle Mesh Ewald (PME) method on large scales. In spite of the success of the model in simulating a pure water system, and extremely dilute ionic solutions [2], periodic boundary conditions create unphysical charge-charge correlations at finite ion concentrations. Although expanding the system size can reduce these extraneous correlations to negligible levels in principle, the PME method remains more efficient on large scales. In this work we re-evaluate the applied boundary conditions at the cavity wall to correct the problem at its origin, which is related to torques on water molecules due to the electrostatic reaction field [3]. Here, we apply a mean field force representing bulk media outside the cavity, and a self-correcting torque field is introduced to enforce translational symmetry. Our modified ICSM maintains accuracy, performs more efficiently than previously and removes the primary cause of spatial correlations between ions.[1] Y. Lin, A. Baumketner, S. Deng, Z. Xu, D. Jacobs, W. Cai, An image-based reaction field method for electrostatic interactions in molecular dynamics simulations of aqueous solutions. J. Chem. Phys. 131, (2009).[2] Y. Lin, A. Baumketner, W. Song, S. Deng, D. Jacobs, and W. Cai, Ionic solvation studied by image-charge reaction field method. J. Chem. Phys. 134, (2011).[3] W. Song, Y. Lin, A. Baumketner, S. Deng, W. Cai and D. Jacobs, Effect of the reaction field on molecular forces and torques revealed by an Image-Charge Solvation Model, Comm. Comp. Phys. 13:129-149 (2013).