At R = 3.4 Å, the face-to-face separation is smaller than the sum of two sulfur atomic radii. Here, charge penetration is significant and E elst < 0 for cofacial stacking and at all lateral displacements. This occurs despite the fact that the total interaction potential is actually repulsive for zero-displacement cofacial stacking. The vdW potential becomes repulsive in the cofacial configuration and its shape is well matched to that of E int . Thus, at all three representative values of R, the vdW picture provides a satisfactory description of the interaction potential for parallel-displacement. Electrostatics is not needed to explain the short-range configurational preferences and undue focus on electrostatics may even lead to erroneous conclusions.