  Evaluating the total energy of an extended distribution of point charges,
which interact through the Coulomb potential, is central to the study of
condensed matter. With near ubiquity, the summation required is carried out
using Ewald's method, which splits the problem into two separately convergent
sums; one in real space and the other in reciprocal space. Density functional
based electronic structure methods require the evaluation of the ion-ion
repulsive energy, neutralised by a uniform background charge. Here a purely
real-space approach is described. It is straightforward to implement,
computationally efficient and offers linear scaling. When applied to the
evaluation of the electrostatic energy of neutral ionic crystals, it is shown
to be closely related to Wolf's method.
