The dispersion energy cannot be modeled accurately using second-order perturbation theory. 151,161-163 Higher-order SAPT exhibits prohibitive O(N 7 ) scaling, 163-165 so XSAPT exploits the inherent separability of eq. 1 to substitute a many-body dispersion (MBD) model for E disp . The result is hybrid method called XSAPT + MBD that achieves sub-kcal/mol accuracy with respect to the best ab initio benchmarks, for both E int and its components. This implies that interaction energies do not significantly benefit from error cancellation. 170 Unlike energy decompositions based on density-functional theory (DFT), where dispersion can be difficult to disentangle from semilocal exchange and correlation, 171-173 the energy components in XSAPT + MBD agree with those from reliable third-order SAPT calculations. All calculations reported herein were performed using Q-Chem. Additional details can be found in the Supporting Information, including a more complete description of the XSAPT + MBD approach. All calcula- tions use the def2-TZVPD basis set, 175 which affords converged results for each energy component. 3 Results and Discussion 3.1. vdW Model for Offset-Stacking. The benzene dimer has long been regarded as an archetypal example of π-stacking, although it is actually an outlier among PAHs. Already for naphthalene dimer, and increasingly so for larger acene dimers, parallel-displaced π-stacking is significantly more stable as compared to C-H• • • π interactions, but for benzene dimer the T-shaped configuration is very close in energy. Electrostatic interactions eventually saturate as the number of aromatic rings increases whereas dispersion and Pauli repulsion do not. Nevertheless, benzene dimer establishes a baseline for evaluation of substituent and heteroatom effects, so we begin with a brief review of the vdW picture as illustrated by energy components for (C 6 H 6 ) 2 along a cofacial phenolbenzene total interaction van der Waals electrostatics sliding coordinate (Fig. ). This analysis is performed separately at face-to-face distances of 3.4 Å and 3.8 Å.