For Ar 2 at R = 3.76 Å, which is close to the equilibrium separation, Fig. (b) quantifies the three terms in Eq. ( ). The attractive and repulsive contributions are each ∼ 90 E h in magnitude yet sum to a mere -0.000164 E h = -35.9 cm -1 , which is the total value of E elst . In this particular case, the increased attraction of E en elst at short range wins out over increased repulsion of E ee elst + E nn elst , although the effect is very small. It may be larger in other systems, as in (C 6 H 6 ) 2 (see Fig. ), although the sign is not guaranteed one way or the other and examples can be found where E elst > 0 at short range. 196 Thus, Coulomb repulsion does not account for steric repulsion. Instead, short-range repulsion originates with the exchange energy (E exch ), as seen for Ar 2 in Fig. . This is Pauli repulsion, originating from the antisymmetry requirement. Within the SAPT formalism, that requirement is not built into the directproduct wavefunction |ψ A 0 |ψ B 0 . This is more than a semantic or a model-dependent distinction, however. Results above demonstrate that interpenetration of two charge densities ρA (r) and ρB (r) need not lead to a repulsive interaction, when the electron-nuclear interactions are considered. Thus, a distinct physical phenomenon is needed to explain steric repulsion.