The quantities PR elec and PR hole measure the size of the exciton in terms of the coordinates of the electron (r elec ) and hole (r hole ), respectively. Their average, PR e-h , thus contributes to overall exciton size along the extracule (center-of-mass) coordinate, r elec + r hole . A complementary metric is the coherence length of the exciton (L coh ), which measures exciton size in terms of the intracule coordinate, 306 r elecr hole . That quantity may be defined using the CT indices according to L coh = (PR e-h )