that the triplet is characterized by more complex electron correlation mechanisms than the quintet spin state and only if these correlation effects are precisely accounted for, the triplet is predicted to be the ground state. A (14, 16) active space erroneously predicts a quintet ground state, even when coupled to the post-CASSCF perturbative CASPT2 correction. If the active space is substantially enlarged, CAS (32, 34), consisting of the entire ligandbased π system, the σ donating orbitals, and the valence and double-shell d orbitals, complex mechanisms like 3 d -3 dπ * excitations are observed in the wave function, 18 that can be ascribed to correlation induced delocalization of metal charge to the macrocycle, a correlated π-backdonation. Only then a triplet ground state is predicted. In a joint FCIQMC and DMRG study, we have also analyzed the CI-expansion of the wave function and the orbital entanglement to visualize the complex correlation mechanisms taking place in this system. 18