The CAS (3,3) calculations yield the first doublet-quartet energy gap of 101.5 cm -1 and the second doublet-quartet energy gap of 641.3 cm -1 (see Figure and Table ). This value is in nice agreement with the experimentally predicted value. However, the energy gap is underestimated to 37.2 and 117.4 cm -1 when dynamical correlations are taken into account through NEVPT2. Therefore, to acquire quantitative accuracy, incorporating the correlation of π-orbitals of the coupler is essential. Consequently, CAS(3,3) active space was expanded to CAS(5,5), CAS(7,7) and CAS(9,9) taking into account the orbitals of the coupler possessing sizable tails on the radical centers. The computed doublet-quartet energy gaps with the consideration of different active spaces are shown in Figure and Table . From the figure, it can be observed that for smaller active spaces, the NEVPT2 provides smaller energy gap values in disagreement with experimental results. However, as the active space is enlarged to CAS(7,7) and CAS (9,9), both the CASSCF and NEVPT2 results become roughly equivalent. The CAS (9,9) active space provides comparable doublet-quartet energy gaps as obtained from CAS(7,7) and in concordance with experimentally observed energy gaps highlighting that this active space is adequate enough to provide sufficient electronic correlations to bring agreement with experimental results. For this reason, we have used CAS (9,9) active space for further calculations on other triradicals.