As a final example of a heteroaromatic ring, we consider (pyrimidine) 2 , with (x, y) potential scans plotted in Fig. for configurations with either parallel or antiparallel dipole moment vectors. In either orientation, the double-minimum characteristic of slip-stacking is evident and emerges from E vdW rather than E elst . The latter exhibits only a single minimum, with no lateral displacement when the monomer dipole moments are parallel. When they are antiparallel, E elst favors sliding the two monomers completely apart. Neither behavior is consistent with the total interaction potential. Once again, electrostatic interactions are neither necessary nor sufficient to explain parallel-displaced π-stacking. This is not to say that electrostatics cannot dictate the relative orientation of one monomer with respect to the other. For example, the potential well for antiparallel stacking in (pyrimidine) 2 is much deeper than the one where the monomer dipole moments are parallel.