(3) In practice, the effect of nuclear motion on the S1 energy ΔES1 is determined by performing a series of excited-state calculations on geometries displaced by Q, which can either be obtained by Monte Carlo sampling (50-100 samples, in which Q for each geometry is a random linear combination of Qi) or by finite differences (which usually involves displacements by +Qi and -Qi for each normal mode i). However, if the exciton binding energy is much less sensitive to geometric changes than frontier orbital energies (i.e., ΔEb << ΔEgap), then the changes in the optical gap due to nuclear motion ΔES1 can be approximated by the change in the fundamental gap ΔEgap. We call this (ΔES1 ≈ ΔEgap) the static binding approximation. It is attractive for two reasons. One, it simplifies the evaluation of effects of nuclear movement. When the harmonic approximation is valid (small Q), ΔEgap can simply be calculated from derivatives of orbital energies with respect to normal modes, dEH/dQi and dEL/dQi, which can be obtained in a single ground-state calculation; 17 in contrast, ΔEbind can only be obtained by many excited-state calculations. Two, the magnitude of orbital energy derivatives enables us to identify the vibrations most strongly coupled to the electronic structure.