The spectral density indicates how strongly vibrational degrees of freedom of the chromophore and solvent couple to the bright optical S 1 transition. In Figure we compare the QM/MM AIMD trajectory results with those from the QUBEKit and revQUBEKit FFMD trajectories, with all trajectories sampled for the 60 ps shown in Figure . All trajectories show the dominant peak at 600 cm -1 , with revQUBEKit giving the highest intensity and QM/MM giving the lowest intensity. Both QUBEKit and revQUBEKit give medium intensity peaks in the range from 600-1300 cm -1 , whereas QM/MM produces peaks with relatively smaller intensity in this range. At 1400 cm -1 the QM/MM peaks increase in intensity, and the QUBEKit trajectories produce a peak at ∼1600 cm -1 , which is of similar intensity as the 600 cm -1 peak. Overall, the QUBEKit and revQUBEKit trajectories produce spectral density peaks with higher intensity for these higher frequency chromophore degrees of freedom compared to QM/MM, which we expect to lead to more vibronic structure in the optical spectrum for the FF methods.