The RMSD of the SS-FNO-IP-EOM-CCSD* method decreases significantly from 1.707 eV to 0.285 eV. The change is small as we go to the aug-cc-pCVQZ basis, and the RMSD error is 0.260 eV. The uncontracted 6-311++g** basis gives a very similar error with a smaller dimension of the virtual space, which is similar to the trend observed by Coriani and co-worker . The core-ionization energy with respect to the experiment in SS-FNO-IP-EOM-CCSD*/unc-6-311++g** method shows a MAD of 0.605 eV and the RMSD value of 0.304 eV. The inclusion of triple excitation leads to a significant improvement over the corresponding IP-EOM-CCSD/unc-6-311++g** level of theory, which shows a MAD and RMSD of 2.037 eV and 1.451 eV, respectively. A major contribution to the rest of the error is due to missing higher-order correlation corrections and relativistic effects. Table presents the statistical analysis of the error in IP-EOM-CCSDT and IP-EOM-CCSDTQ results(taken from ref cite ) for the same test set along with our SS-FNO-IP-EOM-CCSD* results. The one-electron variant of the spin-free exact two-component theory (SFX2C) has been used to include the relativistic effect in the calculations. Although a different basis set has been considered for EOM-CCSDT and EOM-CCSDTQ calculations in reference , one can still get a qualitative trend in the results. It can be seen that the SFX2C-SS-FNO-IP-EOM-CCSD* method shows reasonable agreement with the corresponding SFX2C-IP-EOM-CCSDT method. There is a finite improvement in the results with respect to the experiment when full triples correction is considered in the calculation with SFX2C based EOM-CC method. The error with respect to the experiment in core-ionization energy further decreases with the inclusion of quadrupole excitation in the calculations. The maximum error obtained in the SFX2C-IP-EOM-CCSDTQ method is 0.23 eV. It can be seen that the error due to missing higher-order correlation in the SS-FNO-IP-EOM-CCSD* method for second-row elements is partly compensated by the error due to the missing relativistic effect and SS-FNO-IP-EOM-CCSD* results without the SFX2C correction is comparable to the SFX2C-IP-EOM-CCSDT values. The inclusion of SFX2Ccorrections actually increases the error with respect to the experiment in the SS-FNO-IP-EOM-CCSD* method.