During the transition from S 0 to S 1 , the donor loses 0.81 e in total according to the summed DDEC charges, of which N(5) and S(12) alone contribute 0.45 e. This total charge transfer is close to the assumption that one full electron is transferred between the donor and the acceptor. 15 Also of note are the changes in the σ parameters of the LJ potential, which is one of the key advantages of bespoke force field design. Using a standard, transferable force field, the LJ parameters are determined by the atom type, which does not change upon electronic excitation. However, if we examine N(5), for example, we can see that the electron density on this atom decreases in the S 1 state (atomic charge more positive). This leads to a corresponding reduction in the LJ σ parameter (from 0.312 to 0.294 nm), which is intuitive given the decrease in electron density on this atom. Thus, by allowing both bonded and non-bonded force parameters to respond to the electronic state of the molecule, we should be able to model both structural and host-guest contributions to the emission properties of PTZ-DBTO2.