Here, the CC excitations are visualized for the (left, black) electronic and (right, red) photonic subsystems, where each sub-system has an excitation operator (τα, τn) as well as a shared coupled-excitation operator ταñ. Including infinite excitations, one recovers the full configuration interaction (FCI) limit. Panel (a) is adapted with permission from Ref. approach based on Ref. 147, which uses the optimized effective potential (OEP) approach to generate a simple functional to include electron-photon exchange interactions. However, other discussions related to the recent advances of the scQED-DFT approach can be found elsewhere in the literature. DFT uses the total density, n(r), as the main variable. For cavity QED, the photon provides additional DOFs, notably the photonic coordinate qc , which we will see is hidden in new single-particle (SP) orbitals that can be interpreted as corrections to the original SP states due to the cavity. The DFT equations for the noninteracting Kohn-Sham (KS) system can be written as