The experimental absorption and emission energies of PTZ-DBTO2 in toluene are 3.75 and 2.05 eV, respectively, with a corresponding Stoke's shift of 1.7 eV. Thus, the emission energy (2.11 eV) obtained in this work is in excellent agreement with the experimental value, and the value of the Stoke's shift (1.31 eV) is also closer to the experiment, compared to previous work using standard, transferable force fields (0.66 eV). Since the parameters of toluene should be relatively accurate in all transferable force fields, the difference here is mainly due to the inclusion of molecule-specific bond, angle, dihedral and non-bonded parameters corresponding to the S 1 state of PTZ-DBTO2 in the QUBE force field. In particular, by allowing the bonded parameters of the force field (such as the N(5)-C(2) bond length) to adjust according to the electronic state of the molecule, we potentially capture both the geometric and host-guest contributions to the Stoke's shift. In contrast, previous work has used standard, transferable force field parameters and atom typing, 15 where the S 0 and S 1 states shared the same bond, angle, dihedral and Lennard-Jones parameters, an approach which neglects, in particular, the structural changes to the molecule upon electronic excitation.