Even if the (32, 34) active space describes qualitatively well the necessary correlation mechanisms, dynamic correlation effects exist that go beyond the (32, 34) active space. Semicore correlation has been accounted for by further enlarging the active space, CAS (40, 38), including the 3 s and 3 p orbitals, and ultimately increasing the spin gap to 4.4 kcal mol -1 , at the Stochastic-CASSCF level of theory. Coupled cluster calculations with up to quadruple excitations (CCSDTQ) inside the Stochastic-CASSCF (40, 38) orbital space yielded a spin gap of 4.8 kcal mol -1 . 17 Several methods exist to treat efficiently dynamic correlation effects on top of CASSCF wave functions. However, the list dramatically reduces when a large CASSCF wave function is used as reference. The multi-configuration pair-density functional theory, MCPDFT, is one of the few methods that can be effectively coupled to very large CAS reference wave functions. MCPDFT calculations on top of DMRG-CASSCF (32, 34) active space where performed by Zhou et al. and further stabilized the triplet over the quintet with an estimated spin gap of 12.6 kcal mol -1 . Although this method can be coupled to large CAS wave functions, it is not possible to systematically improve it. Moreover, al-though the delocalization error does not affect the SCF procedure in MC-PDFT, as this is carried exclusively using the preceding CASSCF procedure, it is possible that the delocalization error bias, dependent on the chosen exchange and correlation translated functional, still exists that overstabilizes the triplet spin-state. Another example is the tailored coupled cluster approach (TCC) that performs Full CI within the active space and uses those CI coefficients as fixed amplitudes in the subsequent coupled cluster equations, which are then solved to account for the remaining dynamic correlation. The tailored distinguishable cluster method with singles and doubles (TDCSD) and F12 correction gave a spin gap of 5.8 kcal mol -1 . For the current application, the Stochastic-GAS approach has been applied to build and stochastically solve a large RAS-CI wave function. The converged CASSCF (32, 34) MOs have been used as one-electron basis. The 34 active orbitals have been included in the RAS2 space. The RAS1 space was selected by identifying plateaus in the orbital energy of the inactive orbitals for each irreducible representation and including orbitals above these plateaus. In total 32 doubly occupied orbitals were chosen for the RAS1 space, including the four 3 s and 3 p semi-core orbitals from the metal center and the 28 additional σ-orbitals from the macrocycle. Since there were no well defined plateaus in the orbital energies of the virtual orbitals the RAS3 space was simply defined by an energy threshold of 0.85 E h .