The size of the active space has a major influence on the accuracy of a CASSCF calculation. As such, it is necessary to investigate whether an extended active space can further improve the value of 2J. Fig. shows the 2J value calculated by CAS(2,2), CAS(4,4), CAS (6,6) and CAS (8,8) active spaces. The CAS(4,4) active space is observed to have the best agreement with the experimental value, so it was chosen for all further CASSCF calculations.