This terminology recognizes that van der Waals' modifications to the ideal-gas equation contained the effects of both attractive interactions (E disp ) but also finite molecular size (E exch ). The data in Fig. suggest that E vdW is qualitatively sufficient to explain the origin of offset π-stacking in (C 6 H 6 ) 2 . It arises from a competition between Pauli repulsion, which favors an offset in order to reduce density overlap, and dispersion that favors spatially proximal atoms. Electrostatic interactions are not required at all in order for this geometric preference to emerge. This is not to say that there is no role for electrostatics in π-stacking, or that the vdW potential alone total interaction van der Waals electrostatics sets the stacking parameters. Although the attractive electrostatic contours for cofacial benzene dimer [Fig. 28(b)] have little influence on whether slip-stacking occurs or not, they are essential for obtaining the correct face-to-face separation because they provide a stabilizing interaction at short range, without which one would obtain a stacking distance R min ≈ 3.9 Å for the parallel-displaced configuration, 200 significantly larger than the correct value R min = 3.4 Å.