Structural and energetic effects of truncating long ranged interactions in ionic and polar fluids

The effects of Coulomb potential truncation schemes used in computer simulations of ionic and polar fluids are examined by use of integral equation techniques. A renormalized HNC type equation capable of describing both ionic and polar molecular fluids with truncated interactions is derived and applied to several model systems of interest. Good agreement is found between the integral equation results and Monte Carlo simulations of the same potential for dilute solutions of ions in a dielectric continuum. Very large effects on the distribution functions result from truncation of the electrostatic interaction in dilute systems. Even in comparatively dense systems, unrealistic pair correlations near the cutoff distance result from some of the proposed truncation schemes. The effect of Coulomb potential truncation for a molecular model of pure water is also studied. Significant errors appear in the second neighbor region for commonly used truncation schemes; a simple switching function that zeros the potential and its first derivative yields results closest to the Coulomb potential without truncation.