The cumulant approach outlined here relies on two major approximations: That the system of interest can be described as a two-level system or a combination of two-level systems that are non-interacting, and that the Condon approximation is valid. The CV molecule exhibits two bright low-energy transitions that contribute to the visible spectrum, from HOMO to LUMO (S 1 ) and from HOMO-1 to LUMO (S 2 ) (see Sec. III A). These two excitations are relatively close in energy, meaning that nonadiabatic interactions between the two excited states that go beyond a two-level treatment of the system cannot necessarily be ignored. However, for CV, we find that the two excited states stay sufficiently well separated along the MD trajectory, with transition dipole moments associated with S 1 and as S 2 remaining largely orthogonal, thus indicating only weak nonadiabatic coupling between the two excited states. It is therefore assumed that the optical spectrum of CV can be computed by summing the contributions of independent S 1 and S 2 spectra calculated using the cumulant approach.