  An efficient real space method is derived for the evaluation of the
Madelung's potential of ionic crystals. The proposed method is an extension of
the Evjen's method. It takes advantage of a general analysis for the potential
convergence in real space. Indeed, we show that the series convergence is
exponential as a function of the number of annulled multipolar momenta in the
unit cell. The method proposed in this work reaches such an exponential
xconvergence rate. Its efficiency is comparable to the Ewald's method, however
unlike the latter, it uses only simple algebraic functions.
