Efficient methods for handling long-range forces in particle-particle simulations

A number of problems arise when long-range forces, such as those governed by Bessel functions, are used in particle-particle simulations. If a simple cutoff for the interaction is used, the system may find an equilibrium configuration at zero temperature that is not a regular lattice yet has an energy lower than the theoretically predicted minimum for the physical system. We demonstrate two methods to overcome these problems in Monte Carlo and molecular dynamics simulations. The first uses a smoothed potential to truncate the interaction in a single unit cell: this is appropriate for phenomenological characterisations, but may be applied to any potential. The second is a new method for summing the unmodified potential in an infinitely tiled periodic system, which is in excess of 20,000 times faster than previous naive methods which add Periodic images in shells of increasing radius: this is suitable for quantitative studies. Finally, we show that numerical experiments which do not handle the long-range force carefully may give misleading results: both of our proposed methods overcome these problems. (C) 2000 Academic Press.