The accuracy of the SS-FNO-IP-EOM-CCSD* method depends upon the FNO and natural auxiliary function truncation parameters. To study the impact of different truncation parameters, we have selected a test set provided by Cheng et al. This test set comprises a total of 16 molecules and includes 32 ionization energies of carbon (C), nitrogen (N), oxygen (O), and fluorine (F) 1s orbitals. The experimental values of these ionization energies have been widely reported in the literature . In this study, all the calculations have been performed using density fitting approximation. The maximum error introduced by the density fitting approximation is 0.009 eV (See Table ), which is consistent with the error observed in the density fittingbased implementation of Krylov and co-workers . Therefore, the canonical IP-EOM-CCSD* method with density fitting and CVS approximation has been used as the reference in this study. It should be noted the SS-FNO-IP-EOM-CCSD* approximation as described in the present manuscript is equally applicably to valence ionization energy. However, exploring the performance SS-FNO-IP-EOM-CCSD* for valence ionization energy is outside the scope of the present study.