The most inclusive route for the simulation of ECD spectra is the extension of molecular linear response methods to solids with PBCs. Given the size of the typical chromophores involved, the best balance between cost and accuracy would most likely be obtained with TDDFT. Although this author is not aware of an implementation of these methods for ECD in general-purpose codes, many pieces are already available and the time is mature for such developments. A TDDFT-PBC implementation for 1D periodic systems was proposed by Hirata and co-workers in a stand-alone program, but the focus was only on excitation energies and the method has not been generalized to 3D periodicity. In more recent years, more general implementations of TDDFT have appeared, at least for standard UV/Vis absorption. However, these implementations often rely on the Tamm-Dancoff approximation (TDA), which is more efficient but less reliable, and are limited to the Γ point in reciprocal space, which simplifies the equations as the CO coefficients are real but at the expense of accuracy. The choice of functional is also not as simple as for molecules. Optical activity and excited state calculations typically require range-separated hybrid functionals, because they better describe the long-range density response. However, the treatment of exact exchange is still problematic for PBC codes. Common approaches include the use of pure functionals (without exact exchange), hybrid functionals with exchange only in the short range (like HSE06 ), or approximate procedures to include exchange (e.g., the auxiliary density matrix method, ADMM ). Another issue is the size of the basis set involved. In molecular calculations, large Gaussian-based basis sets with diffuse functions are often recommended for accurate results, but these basis sets are problematic for PBC calculations because of linear dependence issues between replicated cells. For bulk conducting materials, plane wave basis sets are used as they naturally satisfy translational symmetry, but for semicon-ductors or molecular crystals a large number of these functions is necessary for convergence of energy and properties. Therefore, mixed Gaussian/plane wave basis sets can be used, but they have the same linear-dependence issues mentioned above. Thus, although all of these approaches make PBC calculations feasible, it is unclear whether the accuracy will be sufficient for ECD and CPL spectra. For instance, a potential issue is that the ECD signal is much smaller (by 2 or 3 orders of magnitude) compared to regular absorption, and each band is often the convolution of multiple signals with opposite sign. Therefore, relatively small errors on individual transitions could lead to qualitatively incorrect spectra. Nonetheless, these limitations may be overcome with further developmental effort.