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When a molecule is electronically excited, it may pass its excitation energy to a nearby molecule. The electronic coupling is an important parameter in the excitation energy transfer rate. It is now possible to calculate the coupling strength with recently developed computational methods. Zhi-Qiang You and Chao-Ping Hsu on page 102 show that, at a large distance, the coupling is fundamentally a Förster dipole–dipole coupling (as shown in the lower right corner of the cover); and at a short distance, a short-range coupling arises from orbital overlaps. Exchange interaction is noted.