We next use XSAPT + MBD to consider a question that has been asked repeatedly in the literature, namely, do π-π interactions constitute a unique form of dispersion, different from that encountered in other molecules? CCSD(T) calculations on the coronene dimer (C 24 H 12 ) 2 and perhydrocoronene dimer (C 24 H 36 ) 2 , as models of graphene and graphane, respectively, suggest that E int is more than 30% larger per carbon atom in the [24]graphene model as compared to [24]graphane. 264 This effect is not captured in B97 + D/6- Close examination of stacking energies for acene dimers and their saturated perhydroacene analogues suggests an explanation for enhanced stacking energies in [n]graphenes. 148 Figure compares stacking energies for the acene and perhydroacene dimers, where the former are considered in both cofacial (paralleldisplaced) and perpendicular (T-shaped) geometries. For n ≥ 2 rings, the π-stacked acene is systematically more stable than either the C-H• • • π acene dimer or the stacked perhydroacene dimer, by an amount that increases with the number of rings. In contrast, all three structures have very similar interaction energies for n = 1 ring (i.e., the benzene and cyclohexane dimers), demonstrating that results for benzene dimer are not representative of larger PAHs. These differences are confirmed in calculations of C 6 H 6 atop C 96 H 24 , which exhibit a clear preference for cofacial π-stacking over perpendicular C-H• • • π structures. Remarkably, stacking energies for the perhydroacene dimers with one to five cyclohexane rings are a nearly identical match to data for T-shaped acene dimers with the same number of carbon atoms. 148 This suggests that there is something special about π-π interactions. To understand what it might be, we consider isosurfaces of the reduced density gradient (RDG), 266 RDG(r) = 1 2(3π 2 ) 1/3 ∇ρ(r) ρ(r) which has been used extensively as an indicator of noncovalent interactions. This works because noncovalent interactions are characterized by relatively large oscillations in the density (manifesting as large values of ∇ρ(r) ) in regions where the density itself is small, due to disturbances in the tails of the monomer electron densities originating in the presence of the other monomer. Isosurfaces of RDG(r) for the acene and perhydroacene dimers with n = 1 or 2 rings are plotted in Fig. . We observe that the RDG isosurface is corrugated for both the T-shaped acene dimers and the perhydroacene dimers, with stronger interactions when the C-H bonds of one monomer point into the middle of the ring of its partner. In contrast, the RDG isosurfaces for the cofacial acene dimers are relatively flat, meaning that strong interactions persist upon lateral displacement.