The simulation of CPL spectra introduces another challenge beyond those discussed so far: the transition occurs from the minimum structure in the first excited state. Excited state geometry optimizations are rather uncommon with PBC methods, although some examples have appeared. Current simulations of CPL spectra in extended systems rely on the geometry optimization of individual molecular units within the material. This works as long as the chromophore is fairly rigid, and again helicenes are a good example. 12-18 However, many new materials are based on conjugated polymers or floppy chromophores, where the nature of the excited state electron density distribution may be different than that of the ground state. In these cases, the geometry from where the emission occurs may be significantly influenced by the packing of the solid state environment around the chromophore.