1. for extended systems, it allows a well-defined expression for the macroscopic polarization of the system in terms of the induced current-density in the bulk; 2. it allows one to treat the response to transverse fields; 3. instead of looking for frequency-dependent approximations to the exchange-correlation kernel that are nonlocal functionals of the density (as done in TDDFT), one can look for consistent frequency-dependent approximations that are local functionals of the current-density, such as the Vignale-Kohn (VK) functional. Recently, Berger proposed a functional in the context of TDCDFT that can describe excitons in 3D materials. Linear response of 2D materials by contrast remains a challenge in TD(C)DFT. 674 showed that properties can be obtained in this manner (even if not only the density is needed to get the expectation value of the operator). Bauer's approach requires density functionals that are specific for each property and new, property-specific DFAs must therefore be generated -see, for example, ref. 676. This approach of DFT to molecular properties is not only interesting from a fundamental point of view; it can also show how far the present ideas for generating approximations can be taken?or help us understand why our current ideas work for the energy but not for a given property. The first is to better understand the Kohn-Sham potential. We have no rigorous proof that v s , which appears in eqn ( ) and ( ), for example, exists and, to my knowledge, no efficient numerical tool to construct an approximate one exists. Let me try to be a bit more precise.