Truncation of Coulombic interactions in computer simulations of liquids

Several methods are used to take long ranged forces into account in computer simulation of liquids of charged or dipolar particles. These methods include Ewald summation, reaction field methods, and truncation of the potential. Here we investigate various ways of truncating a Coulombic potential to give a finite ranged potential for use in computer simulation. Three truncation schemes, namely the minimum image method, a ‘‘spherical truncation’’ method, and a method we call ‘‘spherical Ewald truncation,’’ are tested by applying them to simulations of an aqueous 2–2 electrolyte solution, liquid KCl, and liquid BeF2. Various ways of calculating the thermodynamic and structural properties for the Coulombic system from the simulation results for the finite ranged truncated potentials are tested and compared. For the dilute electrolyte, the minimum image method and the spherical truncation method (which also includes a shift of the potential to make it continuous) are the most useful truncation schemes. For the two fused salt systems, the spherical Ewald truncated potential is the most useful and gives structures that are very close to those of the full Coulombic potential. The accuracy of the hypernetted chain equation (HNC) for all these systems is tested and found to be remarkably good even for BeF2, which forms a low coordination number random tetrahedral network. The reference hypernetted chain (RHNC) perturbation method for correcting the simulation results for the effect of long ranged forces is also applied to these systems and found to work very well.