where σ i and π i are the single and double bond lengths, respectively (see Fig. ). Figures and show the time evolution and distribution of the BLA along the QM/MM MD trajectory. The BLA is consistently lower for the QM-sampled moiety. The lower the BLA, the more equidistant the single and double C-C bonds are, leading to higher conjugation and hence lower excitation energy, which explains the redshift observed in the OPA spectra for the QM-sampled moiety (Fig. ). The reason for the lower BLA in the MMsampled moiety can only be due to the C-C single-and double-bond distances: The timeline and distribution of the single and double bond distances are shown, respectively, in Fig. and Fig. . C-C single and double bond distances are, respectively, overestimated and underestimated in the MM-sampled moiety, leading to a higher BLA.