Target V (rPSB11) is a natural chromophore that is covalently bound to opsins through a Schiff base linkage. It is extensively studied due to its role initiating the vision process upon light absorption in animals. For all the four previous targets, the reference QM database was obtained exploiting DFT methods. However, to investigate the photoisomerisation of this chromophore, which takes place at the 11-cis double bond through a conical intersection, multiconfigurational strategies as CASSCF/CASPT2 methods are typically used on structures obtained from classical MD simulations. Since the BLA and flexible torsions are known to gov- Table : Top (0 K): RMSD between QM and MM optimised geometries of compound IV in terms of bond lengths, bending angles and dihedrals. Bottom (298 K): Average internal energies (kJ/mol) and statistical descriptors with respect to the expected internal energy at room temperature ( k B T 2 (3N -6)= 86.3 kJ/mol): Maximum Unsigned Error (MUE), maximum and minimum deviation (D max and D min ). < ∆E intra QM > is obtained for ethylene terephtalate at QM along the structures obtained with MD runs carried out at ambient conditions with either general-purpose FFs (GAFF and OPLS ) or JOYCE3.0's QMD-FF. ern such photoisomerisations, it is in fact crucial that the adopted FF reliably describes both these properties and their dynamics. Notably, DFT does not reproduce correct BLA, and post-HF calculations are therefore required to investigate photoisomerization. For this reason, we carry out the JOYCE3.0 parameterisation of target V using MP2. The conjugated polyene scaffold present in the chromophore, responsible for light absorption initiating the photochemical process of typical interest, shows that at MP2 level a delocalised electronic structure, where single bonds are stronger than those in alkanes, and double bonds weaker than those in olefins. This fact not only influences equilibrium distances and force constants, but also torsional profiles, suggesting that double bonds in such a molecule should not be described by the harmonic potential terms, but rather by Fourier series, which enables a better description of the expected large torsional displacements. For these reasons, in addition to the equilibrium geometry and Hessian matrix, the QM database includes ten torsional relaxed scans (see also Section S5.6 in the Supporting Information), divided equally between rotations about single bonds and double bonds (δ n and φ n , respectively, with n = 1 . . . 5).