π-stacking with only minor variations. For the toluene dimer (Fig. ), E elst and E ind are almost negligible and the vdW potential matches the total interaction energy almost quantitatively. The double well in the latter, which is characteristic of offset-stacking, arises from reducing the Pauli repulsion penalty via lateral displacement of 1.2 Å. Toluene dimer serves as a control as we proceed to examine (phenol) 2 and (benzonitrile) 2 , where the substituent is quite polar. For X = OH and CN, we consider geometries in which the dipole moments of the substituents are aligned. This causes the electrostatic term to be repulsive in cofacial configurations, reaching a maximum value E elst ∼ 1 kcal/mol when there is no offset. One of these substituents is electron-donating (Hammett parameter σ p = -0.37 for -OH) and the other is withdrawing (σ p = +0.66 for -CN); 183 nevertheless, the picture is remarkably similar for both (phenol) 2 and (benzonitrile) 2 . The vdW potential exhibits a shallow double-minimum in both cases, because dispersion and Pauli repulsion both assume their extreme values when the offset is zero. For (benzonitrile) 2 , E exch is only slightly less than E elst but is considerably more repulsive than E elst in the case of (phenol) 2 , for which the parallel-displaced value of R is shorter. Meanwhile, E elst is more repulsive in cofacial (benzonitrile) 2 than it is for cofacial (phenol) 2 , opposite the prediction of the Hunter-Sanders rules insofar as an electron-withdrawing substituent would be inferred to reduce the quadrupole moment of the aromatic moiety, and thus reduce the quadrupolar repulsion. In the case of (benzonitrile) 2 , E elst > E exch at all geometries surveyed. In this particular case, it is possible rationalize offset-stacking in terms of E elst + E disp rather than E exch + E disp . The advantage of the latter explanation lies in its generality. Pauli repulsion and dispersion have the same effect at all geometries, leading to a slipstacking double minimum or at least a potential that is very flat for lateral displacements, regardless of the presence or nature of the substituents. The same cannot be said for E elst + E disp . In the examples above, substituent dipole moments were aligned parallel to one another. We next consider (C 6 H 5 NO 2 ) 2 in a geometry where the two NO 2 dipole moments are anti-parallel, as shown in Fig. . Once again, only the vdW potential exhibits a potential minimum indicative of offset-stacking. The role of the vdW potential is even more pronounced in the dipolealigned configuration (Fig. ), where the minimum in E int lies around 1.75 Å in the parallel-sliding coordinate, with R ≈ 3.45 Å, whereas E elst has its minimum at a much smaller value of R and is largely ambivalent to changes in the later displacement. The combination E elst +E disp has its minimum at zero lateral displacement; see Fig. . For both parallel and antiparallel configurations of the NO 2 dipole moments, E vdW is needed to obtain a minimum-energy geometry that resembles that obtained on the E int surface. Thus, the vdW picture provides the best way to understand the geometric preferences of dimers involving C 6 H 5 X.