where ωeg = ⟨U (t)⟩ is the thermal average of the energy gap operator U connecting the electronic ground state g to the excited state e, µ eg is the transition dipole moment between these two states, which is taken as a constant within the Condon approximation, and g n (t) is the n th order cumulant function. Atomic units are used throughout these expressions, unless stated otherwise. If the energy fluctuations of the system follow Gaussian statistics, truncation of the cumulant expansion is exact at second order. The absorption is then determined completely by the second-order cumulant, g 2 (t), which can be evaluated as 21