The decomposition method for ferromagnetic and antiferromagnetic components is first tested on the copper complex. The minimal (2,2) space includes the two magnetically active orbitals forming a single σ-coupling pathway, which upon inclusion of the three oxygen 2p orbitals is expanded to an (8,5) active space, see Figure . A Löwdin orbital analysis of the canonical orbitals reveals that the pair of magnetically active orbitals consists predomi-nantly of singly occupied d z 2 orbitals (89-92%) localized at the two metal centers, with small contributions (2-4%) from p y and p z orbitals on the bridging oxygen, which are necessary to form the σ-superexchange pathway. These orbitals closely resemble those described by Hoffmann using the extended Hückel model, making them well-suited for the intended analysis. Further details of the Löwdin orbital analysis on all complexes can be found in the SI. The energy difference between the gerade and ungerade σ pair is approximately 7400 cm -1 , significantly smaller than the energy gap to the nearest p orbital, which is 32900 cm -1 . As a result, the primary configurations of the two spin states involve only doubly occupied oxygen 2p orbitals. Referring to Hoffmann et al., small geometric changes primarily affect the energy splitting of the σ-pair and consequently influence the J coupling, without significantly altering the exchange or Coulomb interactions. The impact of varying the Cu -O -Cu angle in small steps between 123 • and 131 • on the overall magnetic coupling, as determined by CASSCF, NEVPT2, and BS-DFT calculations, is illustrated in Figure .