As Figure shows, LASSIS is capable of qualitatively reproducing both LASSI [3,3] and CASCI along the entire potential energy curve despite having only 321 model states, compared to 4,644 model states in LASSI [3,3] and 63,504 single determinants in the corresponding CASCI calculation. A small discontinuity in the LASSIS energy in Fig. is observed in the range 2.25 Å ≤ R NN ≤ 2.35 Å. We trace this discontinuity to an energy-level crossing between fragment statelets of different point-group symmetry, as discussed more in Appendix C.