How the unit cell surface charge distribution affects the energetics of ion–solvent interactions in simulations

Abstract

The evaluation of the electrostatic potential in condensed phase simulations normally includes an ‘‘extrinsic’’ contribution that manifests natural imbalances in the surface charge distribution of the microscopic unit cell. Most directly affected are ion–solvent interaction energies, and depending on whether the specific simulation conditions eliminate the extrinsic potential or not, these energies can vary by a considerable amount. This is illustrated by examining simulations of dilute aqueous solutions of Cl− and Fe2+ that use either a cutoff scheme or Ewald summation. It is found that the ion–water potential energy can vary with the type of boundary condition by as much as ≊60 kJ mol−1 for Cl− and ≊800 kJ mol−1 for Fe2+. The difference is exclusively due to the extrinsic potential effect and it is easy to calculate an appropriate correction term.