Figure compares the HOMO/LUMO energies and HOMO-LUMO gaps obtained from DFT/TIP3P and DFT/EFP calculations with those from full-system DFT calculations. Overall, the two DFT/MM schemes worked reasonably well in capturing the frontier orbital energies of OLU -in water (see the left two columns in Fig. ). However, as shown in Figs. and, both DFT/MM schemes failed to accurately capture the LUMO energies of the pHBDI --water complexes (and consequently the HOMO-LUMO gaps). Since the pHBDI --water system features relatively closelying LUMO and LUMO+1, we speculate that in some frames, the LUMO of the system may exhibit a greater degree of chromophoresolvent orbital entanglement than indicated by the values of w ′p A and w ′p E in Table , leading to the inadequacy of the DFT/MM models.