antiferromagnetic coupling between the open-shell electrons of the two fragments. On the other hand, LASSIS optimizes fragment statelets of the charge-transfer type using the downfolded Hamiltonian described in Sec. 3.1.3. This modified Hamiltonian essentially balances the minimization of Eq. ( ) with the maximization of Eq. ( ). Due to this modified Hamiltonian, in LASSIS, the Fe II 3d a orbital remains doubly-occupied in the first four fragment statelets, as shown in Table . These statelets instead donate the electron to the Fe III fragment from the singly-occupied orbitals of the Fe II fragment. Such states, despite having individually higher energies [as evaluated by Eq. ( )], have significantly larger weights in the low-energy LASSI eigenstates. This superior procedure for optimizing the LASSIS fragment statelets permits LASSIS, using 927 model states overall, to reproduce LASSI [1,21], using 14,553 model states, in the (11,20) active space.