One-dimensional profiles of E int and E vdW in Fig. bear much qualitative resemblance to the corresponding quantities in benzene dimer, except that the magnitudes are larger because the overall π-stacking well depth is considerably larger for (annulene) 2 . Also similar to the case of benzene dimer is the fact that the induction energy remains negligible at ≤ 1 kcal/mol, or < 5% of the total interaction energy. What is different in this case is that E elst is no longer negligible and varies from -22 kcal/mol to -1 kcal/mol (Fig. ). Nevertheless, E vdW mimics the overall profile of E int quite well, and no combination of electrostatics with another energy component can do so; see Fig. . As in (benzene) 2 , the slope of E elst along the sliding coordinate points toward eclipsed-cofacial stacking rather than offset-stacking, such that electrostatics provides a driving force that opposes offset-stacking, favoring instead the close-approach of the monomers that enhances charge penetration, which is the origin of short-range electrostatic attraction. The analogous energy profiles for porphyrin dimer are provided in Fig. . Because of the larger separation between the porphyrin monomers (3.7 Å) as compared to that in [18]annulene dimer (3.2 Å), magnitudes of the various energy components are smaller in the present case. The induction energy is negligible (|E ind | < 1 kcal/mol), as it was in other systems consider thus far, but despite being similar in size to (annulene) 2 , for (porphyrin) 2 the electrostatic energy is much smaller, |E elst | 2 kcal/mol. The vdW interaction potential drives an offset-stacking preference with a parallel-displacement of 1.5 Å; see Fig. .