The error in the SS-FNO-IP-EOM-CCSD* method depends upon the FNO thresholds. The convergence of core ionization potential (IP) values for the carbon K-edge of CH3CN is shown in Figure . The calculations were performed using the SS-FNO-IP-EOM-CCSD* and FNO-IP-EOM-CCSD* methods in the aug-cc-pCVQZ basis set. The target state FNO threshold was varied to study its effect on the convergence of the computed core IP values. The natural auxiliary threshold has been set to zero. One can see that the results converge with an SS-FNO threshold of 10 -5 , which selects around 47% of the total virtual space (see Table ). The use of perturbative correction results reduces the error in core IP values, and the results converge at 10 -4 . At the 10 -4 threshold, our SS-FNO truncation scheme selects 22% virtual space, and it leads to an error of 0.004 eV. For the rest of the manuscript, we have used a conservative default threshold of 10 -4. . The FNO-IP-EOM-CCSD* IP values converge at the FNO threshold at 10 -5 . It is evident that the SS-FNO basis exhibits a faster rate of convergence for core IP values compared to the standard FNO basis, particularly at lower thresholds. The application of perturbative correction does not yield significant improvements in the accuracy of core ionization potential (IP) values in the standard FNO-IP-EOM-CCSD* approach. This observation is similar to what has been observed for valence IP values in the IP-ADC(3) method .