Secondary, we consider two charged atoms. Figure (a) shows a schematic illustration of the phase-field modelling of two atoms. To solve this case, we convert two atoms A and B into two densities ρ A and ρ B separately. As result, two problems are obtained, i.e., ρ A under an electric field from atom B (Fig. (a) left) and ρ B under an electric field from atom A (Fig.