Removal of pressure and free energy artifacts in charged periodic systems via net charge corrections to the Ewald potential

A uniform neutralizing background plasma is employed by default in standard Ewald calculations for net-charged systems. We show here that this plasma leads to serious artifacts in both system energy and pressure, which lead to unrealistic behavior. These artifacts are especially critical to simulations where either net charge or volume is allowed to change. To correct these problems we institute a net-charge correction term that consists of subtracting off the Ewald sum for a single particle with charge equal to the net charge of the full system and an optional Born or Poisson–Boltzmann term. This correction decreases pressure artifacts by three orders of magnitude and allows rapid energy convergence as the cell size increases. The correction term is general, in that it applies to systems of any shape or net charge. The net-charge correction was tested for systems in all three phases of matter: gas, solid, and liquid, and found to be markedly superior to standard Ewald in all three cases. In the gas phase, isolated molecular energies are quickly achieved as the cell size increases. In the solid phase the vaporization energy of the NaCl crystal is reproduced using free energy perturbation techniques where a single atom is removed. In the liquid phase, the solvation free energy of Na+ was investigated. It is demonstrated that both the net-charge correction and an energy term twice the size of a Born term is required to obtain optimal free energies of ionic hydration.