Despite the fact that E elst changes sign between R = 3.4 Å and 3.8 Å, slip-stacking is observed at both separations with offsets of 1.4 Å and 1.8 Å, respectively. At R = 3.8 Å, where the quadrupolar picture is valid, the electrostatic penalty for π-stacking is indeed reduced by displacement along a parallel sliding coordinate (Fig. ). However, E elst < 0.5 kcal/mol even in the sandwich configuration where it is maximally repulsive. Meanwhile, the total interaction energy profile (E int ) is a good match to what we have previously called the "vdW potential",