the PBC box-based energy or force are accurate to ε in terms of the number of significant digits. While either energy or force can be used to determine k, preliminary test indicates that it is much more practical to use force as it decays as 1/r 2 . Also, if the system size is large, instead of checking all atoms, a randomly selected subset can be used to determine the number of repeats before starting the simulation. As long as no drastic redistribution or polarization of charges occur, the predetermined number of repeats can be used throughout the simulation.