Most of the calculations of ECD and CPL in materials have been done using the exciton chirality method (ECM), which is based on the exciton coupling between coupled oscillators. 12, This method can be used when a chiral molecule contains two or more "separate" chromophores undergoing electric-dipole allowed transitions; in a molecular material, the coupled transitions can occur between separate molecular units. If the relative orientation of the chromophores and the orientation of the electric transition dipole for each chromophore are known, the ECM provides a fairly simple way to determine the ECD spectrum of the system. This method still relies on quantum mechanical (QM) calculations to evaluate the transition dipoles (or the 3D transition density in more elaborate formulations) for the transitions on individual chromophores. However, the QM calculations are limited to individual molecules (or molecular fragments), bypassing the burden of treating the extended system as a whole. The first limitation of the ECM is that it requires the knowledge of the relative orientation of the chromophores. In part, this is a general issue for chiral materials, e.g., when multiple polymorphs are present in the sample or when large floppy chromophores are used, as discussed more in detail later on. However, it is particularly taxing for the ECM based on molecular calculations because the latter cannot easily recover the packing in a material (using, for instance, standard geometry optimizations) unless it is known by other means (e.g., experiment) or the molecular units are rigid (e.g., helicenes). A more important issue is whether the overall spectrum is dominated by the excitonic process while other processes, like the intrinsic chirality of the chromophore or electron delocalization, can be neglected. This is hard to assess beforehand, which limits the predictive power of the ECM.