The JOYCE3.0 parameterization was carried out on a total of 282 internal coordinates and 142 independent parameters, yielding a standard deviation of 0.15 kJ/mol. A preliminary validation of the resulting QMD-FF is displayed in panel a) and b) of Figure . Indeed, from the overlap of the QM and QMD-FF optimised structures in Figure .a and from the values reported in Table it appears that the QMD-FF reproduces the QM equilibrium geometry with negligible error on bond lengths and angles. The FF vibrational frequencies shown in Figure .b also well correlate with their QM counterparts, confirming that the chromophore is well described in a LHA. Turning to the expected large amplitude conformational dynamics, to further validate our FF and better ascertain the gain in accuracy with respect to general purpose descriptions, the QMD-FF obtained from JOYCE 3.0 is compared to a GAFF model. First, as detailed in Table , we highlight the worse performance of the latter, especially in terms of dihedrals and BLA of the polyene system, which anticipates a less accurate description of the photophysics and photochemistry that one would study on geometries extracted from a CL trajectory. Concerning molecular flexibility, it is evident from the selected MM relaxed scans shown in Figure .c that a general purpose model provides lower δ barriers with respect to JOYCE3.0 (see also Figures selected torsional profiles for GAFF (reddish dashed lines) and QMD-FF (greenish lines) with respect to MP2 profiles (green circles); d) computed (RMS-CASPT2/cc-pvDz) and experimental (digitalised from Ref. [88], black solid line) spectra, obtained for rPSB11in vacuo. All spectra have been normalised with respect to their maximum intensity. Simulated spectra have been obtained either at a static level on sole optimized geometry (VE, vertical dashed line), or through the CEA-VE approach (see the Methods Section and S4 in the Supporting Information), on both GAFF (orange) or QMD-FF (green) trajectories.