First, at the MRCI level we obtained full potential energy curves (PECs) of HfCO arising from Hf( 3 F)+CO(X 1 Σ + ), Hf( 3 P)+CO(X 1 Σ + ), Hf( 1 D)+CO(X 1 Σ + ), and Hf( 5 F)+CO(X 1 Σ + ) interactions as a function of Hf•••C distance. For each potential energy scan, the C-O distance was kept fixed to the experimental bond distance (i.e., 1.128 Å) of the isolated CO molecule. This choice was made to simplify the scans, although we had previously noted backbonding could weaken the CO bond. Hence, we do not attempt to compute fundamental frequencies from these potential energy curves since they would be purely approximate. We nevertheless expect the effect on the bond distance to be modest and comparable across all species and electronic states compared. MRCI calculations were initiated from reference complete active space self-consistent field (CASSCF) wavefunctions. Specifically, the CASSCF wavefunctions were constructed by placing 4 electrons in 6 active orbitals [CAS (4,6)]. At long Hf•••C distances (> 5 Å), the selected orbitals are purely 6s and all 5d atomic orbitals of Hf, corresponding to the four active electrons.