Although the slip-stacking phenomenon arises strictly from E vdW , electrostatics-driven reduction in the minimum-energy separation has general implications for π-π interactions that are explored in Section 3.3.3. Those implications depend critically on the fact that electrostatics is attractive at π-stacking distances, a phenomenon that is not captured-even qualitatively-by low-order multipole expansions. For (C 6 H 5 NO 2 ) 2 , this is shown in Figs. 31, which presents the results of an approximate electrostatics calculation that uses only the dipole and quadrupole moments of the two monomers. These multipolar electrostatics surfaces are qualitatively different from the exact ones. In the parallel-dipole arrangement, the multipolar approximation predicts strong electrostatic repulsion, in geometries consistent with π-stacking [Fig. ], whereas shortrange electrostatics is always attractive when it is computed using full monomer charge densities [Fig. ]. For the antiparallel-dipole arrangement, the multipolar approximation changes sign near zero displacement [Fig. 30(h)], which is also completely inconsistent with an exact calculation [Figs. 30(g)], as there is no repulsive region at all in the latter.