Do Special Noncovalent π–π Stacking Interactions Really Exist?

Van der Waals complexes are formed by almost all neutral molecules, which begs the question as to what is so special about the interaction between parallel stacks of arenes. The term π–π stacking interactions should primarily be used as a structural descriptor for unsaturated systems. Genuine π–π interactions are caused by specific electron correlations that are only at work for short intermolecular distances.