AN OPTIMIZED EWALD METHOD FOR LONG-RANGED POTENTIALS

A modified Ewald method is described for calculating the potential and its gradient for systems with long-ranged interactions and periodic boundary conditions (PBC). Following the work of Natoli and Ceperley, the division between the direct- and reciprocal-space terms is optimized by minimizing the squared deviation of the approximate representation from the exact form. In simulations using this method most of the computational effort is required in performing the reciprocal-space summation. For comparable accuracy, this method requires between one half and one third of the number of reciprocal-space lattice vectors of the standard Ewald technique. In addition our technique requires the choice of only a single parameter which controls the accuracy achieved, rather than the three parameters required in the standard Ewald technique. We give formulae (for the face-centered cubic (fcc), body-centered cubic (bcc) and simple cubic (sc) lattices), which allow for efficient evaluation of the terms in the reciprocal-space summation. (C) 1994 Academic Press, Inc.