For the smaller Co/Mg 9 O 9 cluster, we compared embedded EOM-EE-CCSD-in-DFT calculations with all-atom EOM-EE-CCSD and EE-CC2 results. In contrast, for the larger Co/Mg 25 O 25 cluster, calculations were feasible only with embedded EOM-EE-CCSD. In the embedded EOM-EE-CCSD calculations, we truncated the virtual orbital space using concentric localization. For Co/Mg 9 O 9 , embedded EOM-EE-CCSD calculations were also possible without virtual space truncation. As low-level method, we employed the long-range corrected LRC-ωPBEh 82 functional, which is expected to provide an adequate description for transition-metal molecular magnets. Previous benchmarks have shown that replacing LRC-ωPBEh with other long-range corrected functionals such as CAM-B3LYP does not affect the spin-state ordering nor significantly alters spin-related and magnetic properties (e.g., spin-orbit couplings, spin-inversion energy barriers, and susceptibilities). To Co/Mg 9 O 9 , we also applied the EE-CC2 variant with Cholesky decomposition of the electron repulsion integrals. In all calculations the 6-31G * basis set was employed. Benchmarks on Co, Fe, and Ni-based molecular magnets illustrated that using larger basis sets has a small effect on the spin-reversal energy barrier and the computed macroscopic magnetic properties. Core electrons were frozen in all calculations. Open-shell reference states were computed using unrestricted Hartree-Fock (UHF) theory. However, spin contamination in both reference and target states was minimal, with ⟨S 2 ⟩ values ranging from 3.75 to 3.78 for the quartet states of Co adatoms (Table ). All electronic-structure and property calculations for the clusters were carried out using the Q-Chem program package, version 6.0. 86