We also evaluated the ionization energy (IE) of the single-reference ground state at the CCSD(T) level of theory. The removal of an electron from the doubly occupied 2σ molecular orbital of HfCO (X 3 Σ -) produces the HfCO + (X 2 Σ -) cation. At the CCSD(T) level this IE is 6.974 eV, which is slightly higher compared to the first IE of the Hf atom (i.e., 6.825 eV) and could be due to the slightly ionic Hf +0.53 -[CO] -0.53 charge distribution. No experimental reference value is available for comparison to this quantity, so we will use the CCSD(T) reference value for subsequent evaluation of the accuracy of various DFAs in DFT.