Instead of considering the contributions of individual configurations to the total wavefunction, one can consider the natural orbital occupation numbers (NOONs). The most apparent difference is that the minimal active space has slightly higher occupation numbers for the d x 2 -y 2 orbital and slightly smaller occupation numbers for the π b 1 orbital, consistent with more charge transfer to copper and an increase in open shell character (Figure and S4). The radical character in the wavefunction can also be quantified using the NOONs by first assigning bonding and anti-bonding pairs of orbitals and then computing the effective bond order (EBO),