Conversely, the improvement is less pronounced in the standard FNO-IP-EOM-CCSD* method, which exhibits a MAD of 0.166 eV. This trend in the statistics for the FNO-IP-EOM-CCSD* method is consistent with the convergence behaviour observed for CH3CN in Figure . Similar patterns are observed for other statistical parameters as well. At a 10 -5 threshold, the SS-FNO and standard FNO-IP-EOM-CCSD* method gives almost similar errors for core-ionization energy. Therefore, a threshold of 10 -4.5 in the SS-FNO-IP-EOM-CCSD* method gives the best compromise between accuracy and efficiency and is for all the subsequent studies in the paper.