“…First, for electronic degrees of freedom, some of these approaches represent the electronic density in real space, 27,31,36,65,66 whereas others use atom-centered Gaussian basis functions. 19,29,30,45 Second, photon degrees of freedom can be represented directly in real-space 20,24,67 or in Fock space 19,[29][30][31]45 (in a basis of photon-number states). Third, as with standard TDDFT, both real-time 24,31 and linearresponse 19,27,29,30,66 formulations have been put forward.…”