A strategy for charged impurities has been developed by Tölle et al. . Based on the density-embedding idea, a screening potential was defined in that work which, for the example of an ionized molecule as an impurity within a unit cell, replaces the electrostatic effect of the periodic impurity images by the potential of the neutral, non-ionized system. Treating such charged impurities is not straightforwardly possible with conventional methods under periodic boundary conditions. Recently, the ionization potential of water in water has been studied using this approach .