In a series of publications, we have used XSAPT + MBD calculations to explore the fundamental nature of π-π interactions, which have also been investigated using conventional SAPT. Although the benzene dimer has long been the archetypal model system for studying such interactions, it is not a good model for larger polycyclic aromatic hydrocarbon (PAH) dimers, 148 for reasons discussed in Section 3.3.2. Nevertheless, understanding substituent effects on stacking energies in dimers of benzene and small heterocycles remains an important guide for synthetic chemistry. Below, we use XSAPT + MBD to explore the origins of parallel-displaced π-stacking (Section 3.3.1) and then address an old question of whether π-π interactions are a unique type of interaction that is distinct from "ordinary" dispersion (Section 3.3.3).