energy (kcal/mol) We pause here for an aside on nomenclature and attribution. In the supramolecular organic chemistry literature, the quadrupolar electrostatics idea described above is often used synonymously with the term "Hunter-Sanders model". (See Ref. 200 for copious literature citations to support this equivalence.) Although widely repeated, this misrepresents the original force-field model developed by Hunter and Sanders to explain the configurational preferences of (C 6 H 6 ) 2 and other systems. Nevertheless, the idea that π-π interactions have a quadrupolar electrostatic basis remains widespread in the synthetic chemistry literature, 200 as evidenced by recent reviews. While it may be ubiquitous, this quadrupolar picture is also irredeemably incorrect because short-range charge penetration leads to a qualitative breakdown in the multipole approximation at typical vdW contact distances, as discussed in Section 3.1.1. In particular, this is an issue at typical face-to-face separations of 3.4-3.8 Å that are characteristic of π-stacking interactions (see Fig. ). Not only is the multipole expansion divergent as R → 0, but leading-order molecular multipole approximations, and even distributed (atom-centered) multipole expansions may afford the wrong sign for E elst at these intermolecular distances.