In Fig. , we compare the performance of LASSI with its theoretical limit LAS-CASCI, under two variations. We first vary r and q for the (11e,10o) active space. LASSI[0,1] includes no charge transfer rootspaces and only ground state eigenfunctions of the projected Hamiltonian in each fragment (equivalent to (a)-(e) rootspaces in table 2.2). This generates only enough model states to ensure that the LASSI eigenstates are eigenfunctions of the Ŝ2 operator and wrongly predicts ferromagnetic interaction (J > 0) as compared to LAS-CASCI due to the complete omission of kinetic exchange. As r and q increase, the agreement between LASSI and LAS-CASCI improves. The correct negative sign of J is observed for r > 0; q > 4. At LASSI [1,5] (Fig. : green curve with upside down triangles), we observe agreement within CASCI to within 1 cm -1 , from a wave function built from 250 model states.