To understand bonding in HfCO, we start by analyzing the electronic structure of the individual Hf atom and CO molecule. The 3 F ground electronic state of Hf has a valence 5d 2 6s 2 electron configuration. Simple electron promotions within the 5d shell produces its first two excited electronic states, which are experimentally only 16-26 kcal/mol higher in energy (i.e., 3 P at 15.8-25.7 kcal/mol, 1 D at 16.1 kcal/mol). As a result of the low-lying electronic states of the isolated atom, Hf is expected to form complexes with a variety of chemical bonding configurations. We investigate the reaction between the aforementioned electronic states of Hf with the ground state of the CO ligand (X 1 Σ + ). The first excited state of CO (a 3 Π) lies well separated from the ground state (by 139.2 kcal/mol) and hence its interaction with Hf was not pursued in this work. The combination of 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 + ) produce 7 triplet, 3 triplet, 5 singlet, and 7 quintet spin electronic states of the HfCO molecule, respectively. In total, we have studied 10 triplet, 5 singlet, and 7 quintet spin PECs of HfCO at MRCI level of theory to identify its low-lying electronic states (Figure ). Note that all quintet states except b 5 Δ are high in energy and are not shown in Figure .