Figure 4. Example: the variation method is used to determine energies of coupled excited states (ε1-4 and ε*1-4) of the Fenna-Matthews-Olson light-harvesting complex (shown in shown in the inset). The method helps to determine major contributions of uncoupled localized excitations of bacteriochlorophyll molecules (BCL 401-407) to the coupled states. Reproduced from Ref. 2.