The two DFT/CIS methods afford consistently larger errors at the carbon K-edge as compared to the oxygen K-edge. this results from the ∆ε correction in Eq. ( ), where ∆ε = 0.025ε is a somewhat better correction for oxygen than it is for the carbon. In our view, these slightly larger errors for carbon (and in certain cases, for nitrogen) are not large enough to warrant reparameterization. The present version improves upon both uncor- rected CIS and also TD-DFT (using the same functionals) by ∼ 10 eV. The shift ∆J(ε) in Eq. ( ) matches ∆ε very well for second-row elements so results for B3LYP/ CIS are similar to those for CAM-B3LYP/CIS. Note the comparison between mean absolute errors (MAEs) in Table , which are assessed relative to theoretical best estimates of the transition energies. Conventional TD-B3LYP and TD-CAM-B3LYP calculations improve upon conventional CIS by ∼ 1 eV, which is similar to what is observed for valence excitation energies, 10 yet this is much smaller than the ∼ 10 eV improvement for DFT/CIS. This suggests that most of the improvement in the DFT/CIS case comes from the shift ∆ε or ∆J(ε), or in other words that most of the error in TD-DFT excitation energies for K-edge transitions comes from error in the 1s eigenvalues. At the same time, we note that CIS errors are opposite in sign from those obtained using TD-B3LYP and TD-CAM-B3LYP, with CIS overestimating every single K-edge excitation energy while these two TD-DFT methods underestimate every single one. Since there is no SIE in the Hartree-Fock eigenvalues, these discrepancies in the CIS case must result from lack of sufficient orbital relaxation, with only ∼ 1 eV of the error arising from missing electron correlation. That the TD-(CAM-)B3LYP errors in K-edge energies range from -10 eV to -15 eV, while also missing orbital relaxation, suggests that the SIE in 1s orbital energies likely exceeds 20 eV.