The quality of the parameterised QUBE force field is first examined by comparing quantities including normal mode frequencies, dipole moments, optimised structures and potential energy surfaces around key dihedrals, with QM data. Figure shows the vibrational normal mode frequencies of PTZ-DBTO2 in the S 0 and S 1 states from QM optimisations plotted against frequencies obtained following MM optimisation using the QUBE force field. The mean unsigned errors (MUE) between MM and QM calculated frequencies are 25.7 and 25.2 cm -1 (4.0 and 3.5%) for S 0 and S 1 states, respectively. The agreement, here, with QM is actually slightly better than the expected accuracy (6.3%) of the modified Seminario method, which has been benchmarked previously for small organic molecules in the ground state. Table compares the dipole moments calculated from the DDEC atoms-in-molecule point charges, that are used to build the QUBE force field, with the underlying QM data. Recall that the atoms-in-molecule charges are computed from the QM electron density in implicit solvent (PCM), which tends to increase molecular polarization compared to vacuum. Table 1 shows that the dipole moments computed using the DDEC point charges are in excellent agreement with the QM dipoles in PCM. In cases where there is strong anisotropy in the partitioned atomic electron densities, we would not necessarily expect such good agreement between the point charge model and QM, but off-centre charges can be used to account for this. However, given the agreement between QM and QUBE dipole moments, we judged that these were not required here.