As the number of degrees of freedom (i.e., holes in doubly occupied MOs, particles in virtual MOs
of the CASSCF wavefunction) increases, one generates the successive DDCI-1, DDCI-2 and DDCI-3 CI spaces as discussed in the literature for related compounds[25]. To eliminate the arbitrariness of
the set of MOs in the DDCI calculations, natural orbitals were first
generated by averaging the DDCI-1 density matrices of the singlet
and triplet states. This procedure was iterated until convergence
upon the energies. DDCI-3 calculations were performed using the
resulting set of MOs.