3.1.1. Benzene Dimer. Figure shows potential surfaces for (benzene) 2 as one monomer slides along the other at a fixed intermolecular separation of 3.4 Å, corresponding to the parallel-displaced minimum-energy structure. Previous work suggests that the offset-stacking phenomenon in this system (as well as larger acene dimers) can be understood as a competition between Pauli repulsion (E exch ) and dispersion (E disp ). The latter favors placing atoms in close proximity, due to its rapid R -6 decay with atom-atom distance, but exchange-repulsion introduces a significant penalty for eclipsed-cofacial stacking because this aligns the density maxima that occur at the positions of the nuclei. The competition between these two forces is encapsulated in a "van der Waals" (vdW) energy,