As shown in Fig. , the LASSIS potential energy curve experiences a discontinuity in 2. The magnetic coupling constant predicted by LASSI for different q values and r=1, and by CASCI in the (11,10) and (11,20) active spaces. LASSCF (11,10) and LASSCF (11,20) optimized orbitals were used for the corresponding LASSI and CASCI. Data for q other than 5 or 21 [in the (11,10) and (11,20) active spaces respectively] are taken from Ref. 19.