As discussed in Ref. 19 and as shown in Fig. , the LASSI[1,q] method with the smaller (11,10) active space converges quickly to the CASCI limit with respect to q, reaching qualitative agreement by q = 5. However, the same approach applied to the (11,20) active space converges much more slowly, and fails to even predict overall antiferromagnetic (J < 0) interaction until q > 10. In both this work and Ref. 19, this system is the sole example of any LASSI[r,q] test calculation in which q ≈ the number of orbitals in a fragment is not qualitatively converged. We must therefore analyze this case more closely. We first note that in either active space, the high-spin state is always qualitatively single-determinantal and its energy is insensitive to q: its total energy varies by up to ≈ 40 cm -1 between q = 1 and q = 21, whereas the total energy of the low-spin state varies by ≈ 300 cm -1 . Therefore, the slow convergence of J with respect to q reflects mainly the slow convergence of the low-spin total energy. K (∆N, S) given by Eq. ( ), in which S = 2 + ∆S for the Fe II fragment and S = 2 + ∆S for the Fe III fragment] within the different populated Hilbert spaces in each fragment averaged over the lowest five eigenstates [I = 0, 1, 2, 3, 4 in Eq. ( )] of the LASSI [1,21] calculation of the aluminum-substituted MOF node in the (11,20) active space.