The smaller separation is consistent with the paralleldisplaced minimum-energy structure (Fig. ) whereas R = 3.8 Å is the center-to-center distance in the D 6h "sandwich" structure (Fig. ), which is a saddle point between equivalent parallel-displaced minima. At 3.4 Å separation, equal to the sum of the atomic radii of two carbon atoms, 177 electrostatics is attractive in the cofacial arrangement (E elst < 0), despite a leadingorder quadrupolar term that is repulsive. In contrast, E elst > 0 for R = 3.8 Å, as this separation is sufficient to admit a multipolar description. 144 Analysis at 3.8 Å separation has been used to justify the Hunter-Sanders hypothesis, 137 yet that distance lies (just) outside of typical vdW contact distances and is not representative of π-stacking. (Note that the interlayer separation in graphene is 3.35 Å. 178 ) Electrostatic repulsion characterizes less than 3% of π-π contacts in proteins, so any analysis based on R = 3.8 Å separation is questionable as a basis for understanding π-π interactions.