One can have additional computational gain by using the R3CVS approximation. Figure presents the error distribution plot with respect to the canonical values for Core IP values with and without the R3CVS approximation. The corresponding statistical parameter with respect to canonical IP-EOM-CCSD* method is presented in Table . It can be seen that using R3CVS approximation, the MAD value increases from 0.08 eV to 0.14 eV at 10 - 4.5 threshold. The RMSD value is 0.09 eV, which also increases with respect to the corresponding SS-FNO-IP-EOM-CCSD* value without the R3CVS approximation. Figure represents the error distribution plot of R3CVS-SS-FNO-IP-EOM-CCSD* method. It can be seen that the systematic improvable nature of the SS-FNO is still retained with the R3CVS approximation. Coriani and co-workers have previously shown that the use of a frozen core approximation leads to small error in the core excitation energy with a significant increase in the computational efficiency. Similar results are observed in the present case also, where the coupled cluster amplitudes for the core orbital in R3CVS-SS-FNO-IP-EOM-CCSD* are replaced with MP2 amplitudes, which we call MP2-Core-CVS approximation. Table presents the statistical analysis of error in MP2-Core-R3CVS-SS-FNO-IP-EOM-CCSD* method. It can be seen that the use of MP2-Core-CVS approximation leads to small additional error with respect to the canonical values over and above which is observed.