Simplistic Coulomb forces in molecular dynamics: Comparing the Wolf and shifted-force approximations

This paper compares the Wolf method to the shifted forces (SF) method for efficient computer simulation of isotropic systems interacting via Coulomb forces, taking results from the Ewald summation method as representing the true behavior. We find that for the Hansen-McDonald molten salt model the SF approximation overall reproduces the structural and dynamical properties as accurately as does the Wolf method. It is shown that the optimal Wolf damping parameter depends on the property in focus, and that neither the potential energy nor the radial distribution function are useful measures for the convergence of theWolf method to the Ewald summation method. The SF approximation is also tested for the SPC/Fw model of liquid water at room temperature, showing good agreement with both the Wolf and the particle mesh Ewald methods; this confirms previous findings [Fennell & Gezelter, J. Chem. Phys. {\bf 124}, 234104 (2006)]. Beside its conceptualsimplicity the SF approximation implies a speed-up of a factor 2 to 3 compared to the Wolf method (which is in turn much faster than the Ewald method).