Multiple charge spreading as a generalization of the Bertaut approach to lattice summation of Coulomb series in crystals

The Bertaut approach associated with charge spreading so as to enhance the rate of convergence of Coulomb series in crystals is extended to the case of an arbitrary multiple spreading with a given initial spreading function. It is shown that the effect of spreading may in general be treated as a uniform transformation of space, providing that zero mean potential as a universal spatial property is sustained. As a result, electrostatic potentials driven by different orders of multiple spreading can be obtained from the same energy functional in a consistent manner. It is found that the effect of multiple spreading gives rise to more advanced forms described, for example, by simple exponential decrease, but the functional description based on a Gaussian spreading turns out to be invariant.