for the attractive contribution. Signs in Eq. ( ) reflect the convention that electron densities are output from electronic structure programs as strictly positive quantities, representing probability densities of negatively charged electrons. Although E nn elst and E ee elst become increasingly repulsive at shorter intermolecular separation, E en elst becomes more attractive as the separation is reduced and the electrons in ρ A (r) gain better access to the nuclei on B, and vice versa.