Other similar forms of the TD-DFT and CIS equations have been derived for the scQED scheme, such as those presented in Ref. 99 and Ref. 149, respectively, and both approaches yield similar results. In principle, in all methods discussed until now, an arbitrary number of photonic basis states can be included in order to converge the photonic contribution with little-to-no increase in overall expense due to the relative simplicity of the photonic subsystem compared to the electronic one. Although, more work is needed to test the results against an increasing number of photonic basis states when using these QED approaches, whereas most of the work usually only includes the vacuum |0⟩ and |1⟩ Fock/number states. In the previous section on wavefunction-based methods and specifically with the QED coupled cluster methods, more work has been done to test such convergence. However, due to the method itself, the expense becomes too large to include more than a couple photonic states yet still does not capture the complete physics in comparison to a full configuration interaction (FCI) approach for model systems.