The magnetic exchange in the CAS(2,2) active space was calculated to be 72.3 cm -1 , underestimating the experimental value. This indicates that the minimal active space lacks the necessary static and dynamic correlations, entailing a higher active space. The next choice is to utilize a CAS(4,4) active space, but choosing the correct orbitals from the set of occupied and virtual orbitals is challenging due to the extreme sensitivity of the system to the active space. The correct active space should provide a large portion of the necessary electron correlation in the system. So when we have the correct active space, the triplet-singlet excitation predicted by the CASSCF+NEVPT2 result should only have a minimal difference from the CASSCF excitation energy. Based on this principle, we have devised a method of active space scanning that involves successively changing the occupied and virtual orbitals in the active space till the previously mentioned criterion is satisfied. To carry out the scanning procedure for CAS(4,4), three orbitals were fixed in the active space (the two SOMOs and either an occupied or virtual orbital), and the fourth orbital was chosen from a set of seven consecutive orbitals. Firstly, the HOMO and the two SOMOs were fixed in the active space, and calculations were performed by successively changing the virtual orbitals (Fig. ). Then the LUMO and two SOMOs were fixed and occupied orbitals were varied (Fig. ). We observe that among the various orbitals, HOMO+(2 SOMO)+(LUMO+6), (HOMO-6)+(2 SOMO)+LUMO and (HOMO-7)+(2 SOMO)+LUMO produce the least difference between CASSCF and CASSCF+NEVPT transition energies with differences of 7.6 cm -1 , 10.3 cm -1 and 2.3 cm -1 respectively. The three active spaces also have good agreement with the experimental value. Among the aforementioned active spaces, (HOMO-7)+(2 SOMO)+LUMO has the least difference and predicts a 2J value of 118.0 cm -1 which is the closest to the experimental value.