For the [2Fe-2S] 2-complex, both LASSIS and LASSIS-PDFT predict the singlet state (S=0), i.e. antiferromagnetically coupled Fe III centers as the ground state. The energy gap between the highest (S=5) and the lowest spin-state (S=0) is 1735.2 cm -1 at LASSIS and 3141.7 cm -1 at LASSIS-PDFT, almost twice of the LASSIS predicted value. As shown in Table , LASSIS and LASSIS-PDFT results are consistent with DMRG and DMRG-PDFT, differing by < 10% at each spin state. However, the successive gaps between spin-states widen with the inclusion of the dynamic correlation.