High Quality Content by WIKIPEDIA articles! Ewald summation, named after Paul Peter Ewald, is a method for computing the interaction energies of periodic systems (e.g. crystals), particularly electrostatic energies. Ewald summation is a special case of the Poisson summation formula, replacing the summation of interaction energies in real space with an equivalent summation in Fourier space. The advantage of this approach is the rapid convergence of the Fourier-space summation compared to its real-space equivalent when the real-space interactions are long-range. Because electrostatic energies consist of both short- and long-range interactions, it is maximally efficient to decompose the interaction potential into a short-range component summed in real space and a long-range component summed in Fourier space.