System not charged neutral,

The energy diverges for non-neutral charged systems with periodic boundary conditions. However, you can assume the net charge of the system is neutralized by a uniform background plasma at the infinite boundary of the Ewald sum, and a correction to the system energy can be applied to reduce artifacts. See J. Chem. Phys. 108, 7070 (1998); http://dx.doi.org/10.1063/1.476320