This strong SOC leads to a large zero-field splitting of the ground state into eight pairwise degenerate magnetic levels (Figure ), whose energies are listed in Table . This spinorbit splitting closely resembles that observed in the Co(II)-SMM. From the magnetic sublevels of Co/Mg 9 O 9 and following Eq. ( ), we obtain a spin-inversion energy barrier of 427 cm -1 using EOM-EE-CCSD, which agrees well with embedded EOM-EE-CCSD results, with differences of only 10 cm -1 . These values are in close agreement also with that extracted from an inelastic electron tunneling spectroscopy (IETS) measurement (468 cm -1 ) 54 and with the spin-inversion barrier computed for the Co(II)-SMM using embedded EOM-CCSD (469 cm -1 ). By moving from the C1 to the C2 partitioning scheme and expanding the cluster size from Co/Mg 9 O 9 to Co/Mg 25 O 25 , we reaffirm that the C2 partitioning provides a more accurate representation of the all-atom model, yielding spin-inversion barriers in closer agreement with EOM-EE-CCSD, whereas the impact of the cluster size remains minor.