Figure 1.

A_i and A_j have been selected to show the description of atom-atom charges based on the atom-atom vectors R_ij and ${\mathbf{R}}_{\mathit{\text{ij}}}^{\prime }$, whose atom-atom charges are described as ${\rho }_{A_i,A_j}^{12}(\mathbf{r}-{\mathbf{R}}_{\mathit{\text{ij}}})\text{and}{\rho }_{A_i,A_j}^{12}(\mathbf{r}-{\mathbf{R}}_{\mathit{\text{ij}}}^{\prime })$ respectively.