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 sta ndard Ewald calculations for net-charged systems. We show here that th is plasma leads to serious artifacts in both system energy and pressur e, which lead to unrealistic behavior. These artifacts are especially critical to simulations where either net charge or volume is allowed t o change. To correct these problems we institute a net-charge correcti on term that consists of subtracting off the Ewald sum for a single pa rticle with charge equal to the net charge of the full system and an o ptional Born or Poisson-Boltzmann term. This correction decreases pres sure artifacts by three orders of magnitude and allows rapid energy co nvergence as the cell size increases. The correction term is general, in that it applies to systems of any shape or net charge. The net-char ge correction was tested for systems in all three phases of matter: ga s, solid and liquid, and found to be markedly superior to standard Ewa ld in all three cases. In the gas phase, isolated molecular energies a re quickly achieved as the cell size increases. In the solid phase the vaporization energy of the NaCl crystal is reproduced using free ener gy perturbation techniques where a single atom is removed. In the liqu id phase, the solvation free energy of Na+ was investigated. It is dem onstrated that both the net-charge correction and an energy term twice the size of a Born term is required to obtain optimal free energies o f ionic hydration. (C) 1998 American Institute of Physics.