An intermediate step between exciton models and full QM calculations is represented by tight-binding models. Tight-binding models build an ad hoc Hamiltonian that only contains interactions between nearest-neighbors. Exciton and tight-binding models were also combined to allow calculations when computational resources were still limited. These models are very computationally efficient, but they are system-specific and they require reference data for the parametrization of the Hamiltonian. Furthermore, longer-range interactions may be important when excited state properties are involved, especially with a sensitive property such as optical activity. Another approach is to neglect the response of the ground state electron density to the external field; 58 this corresponds to approximating excited states as single Slater determinants built from the COs, so that the excitation energies are just orbital energy differences and the transition moments are computed between individual Slater determinants. Although this strategy is also computationally efficient because it sidesteps the linear response calculation, it is not typically a good approximation to obtain excited state energies from differences in orbital energies (otherwise TDDFT would not be so popular). From this author's experience with OR, the orbital response accounts for about half of the property magnitude; considering the relationship between OR and CD, see Eq. 10, it seems unlikely that this approach can provide reliable results in general.