Initially, we determined the pre-edge energies of different unsubstituted complexes using two different functionals, B3LYP and PBE0, paired with two different relativistic corrections, namely, DKH and ZORA. The calculated pre-edge energies utilizing these functionals and corrections are collected in Table . For B3LYP functional and DKH relativistic correction, a linear shift in the range of 87-93 eV was necessary to align with experimental energies. This linear shift decreased significantly to 16-32 eV when applying the ZORA correction instead. Furthermore, when using the PBE0 functional with both DKH and ZORA corrections, the required shifts were reduced further to 80-86 eV and 10-25 eV, respectively. It was observed that the required linear shift generally increased for elements down the group from Mn to Co when using the DKH correction; however, it decreased when using the ZORA correction (Table ). Thus, PBE0/ZORA/ZORA-def2-TZVP is optimal for achieving the absolute peak energies closest to the experimental results with minimal linear shifts. The combination of the ZORA correction with the def2-TZVP functional has previously been demonstrated to effectively predict S and Cl K-edge spectra by Neese et al. Nevertheless, notable deviations emerge between the IWAE peak derived from TDDFT calculations and the corresponding experimental peaks. α and 50% β 1s electron to 3d z 2 and 3d x 2 -y 2 respectively (Figure ). The same orbitals are involved in the transition for Co[Cor](py) 2 however, the intensity of the first transition is very small in comparison to that for the PPh 3 analog (Figure ). The Co[Por](py)Cl involves the transition of 50% α and 50% β 1s electron to 3d z 2 (Figure ).