In biochemical contexts, πstacking often involves heterocycles ranging from purines and pyrimidines in DNA to histidine and tryptophan in proteins. The pyridine dimer and the heteromolecular (pyridine)• • • (benzene) dimer display an even stronger preference for parallel-displaced stacking as compared to the benzene dimer, 184,185 and we next examine dimers of pyridine, pyrimidine, and thiophene. These might be anticipated to afford enhanced electrostatic interactions (even in the absence of substituents) due to the monomer's dipole moment. In fact, E elst remains rather small as seen from the one-dimensional energy component profiles for (pyridine) 2 and (thiophene) 2 in Fig. .