The second variation explored is the active space dependence of J. For all three active space choices, agreement of J to within 3 cm -1 was achieved with vastly fewer model states than the number of single determinants in the corresponding CASCI: 7425 model states for the LASSI [1,15](11e,20o) calculation, as compared to ≈ 6 × 10 8 determinants in the CASCI(11e,20o) wave function. Clearly, the convergence with respect to q is less rapid in the larger active space. However, we note that the von Neumann entropies [Eq. ( )] for the LASSI [1,15](11e,20o) (max(s KP ) = 0.95) results are similar to the LASSI [1,5](11e,10o) (max(s KP ) = 0.95) results. (See supporting information for a complete list.) This means that the entanglement between the two fragments does not increase in the larger active space, suggesting that the higher required q in the larger active space is due rather to the less-accurate modeling of each fragment individually by the LASSI model states constructed from the eigenstates of ĤPK .