The sum of the atomic radii for two sulfur atoms is ≈ 3.6 Å, 177 so at R = 4.3 Å the monomers are relatively well separated and there should be little charge penetration. In this case, E elst is repulsive across the range of lateral displacements, consistent with dipole-dipole and quadrupole-quadrupole repulsion. Although the total electrostatic interaction does not exceed 0.5 kcal/mol at this separation, that is almost as repulsive as E exch , whose magnitude falls off rapidly with distance and is much diminished at R = 4.3 Å. Nevertheless, the slip- stacked configuration is more favorable than cofacial stacking even at this large distance. The shallow doubleminimum eliciting that behavior is best approximated by E vdW , which is also shallow to the point of being nearly flat in the region of ±1.8 Å displacement from the cofacial saddle point. When electrostatics goes away almost entirely, at R = 3.9 Å, then E int ≈ E vdW almost quantitatively (Fig. ).