In fact, Eq. ( ) defines exactly the same Hamiltonian as was used to obtain the Kth fragment's reference state originally in the parent LASSCF calculation, but in Eq. ( ) it is diagonalized in a different spin sector. This makes it more straightforward to conceptualize the spin-flipped fragment statelets as "excitations" and to combine them across different fragments into large numbers of model states. On the other hand, it is possible that freezing all other fragments in their reference state omits some significant relaxation effects that are included in LASSI [r,q]. This is a potential area of future investigation.