Going from former to latter would have been difficult with the non-embedded method due to large scaling, but with embedding, we were able to use the larger basis set, also including the triple corrections and relativistic effects. of the inherently localized nature of the core-ionization process. DMET-EOM-CC with an optimal embedding space, produces minimal systematic error (MUE 0.1 eV) and a standard deviation of about 0.1 eV relative to unembedded method. This is demonstrated with 82 core-ionization energies of N-and O-core amino acids. DMET-EOM-CC reduces the cost of the calculations compared to conventional EOM-CC by many orders of magnitude. With DMET-EOM-CC, it was possible to compute the core-ionization of some of the extended systems like uracil hexamer, azafullerene, and cholorphyll a, which would not be affordable with conventional EOM-CC.