well by the meta-GGA hybrid M06-2X with less than 8% error. This observation is in line with our past DFT analysis of HfB, which had almost identical De predictions for M06-2X and CCSD(T). Although it should be noted in contrast to HfCX (X 3 Σ -) and HfB (X 4 Σ -), which have unpaired valence electrons, M06-2X was previously observed to perform the worst at predicting De for the closed-shell ground state of HfO (X 1 Σ + ). For both HfCS and HfCSe, the two bestperforming functionals of HfCO (B3LYP and ωB97X) also provided small errors (~5-8%), reinforcing our suggestion of the use of these functionals. Notably, all the studied functionals preserved the linear De vs. μ trend that we observed earlier (Figure and ESI Figure ). At first the agreement of CCSD(T) and B3LYP on HfCX dissociation is surprising. For example, according to the work by Wilson et al., B3LYP underestimated dissociation of CO from coordination complexes with first row transition metals, including Fe(CO)5 and Cr(CO)6, and this trend was reversed in FeCO, where B3LYP overestimated dissociation. Nevertheless, it was suggested B3LYP predicted a higher error for the dissociation with Fe due to stronger multireference character of this system, whereas HfCO studied here is single reference. In our previous work we observed B3LYP errors of 7 and 35.4% for dissociations of single-reference ionic HfO (closed-shell X 1 Ʃ + ) and HfB (X 4 Ʃ -) systems, respectively. Note that it is rather difficult to come to a firm conclusion of which family or functional is best for all Hf-complexes especially if the degree of metal-organic bonding is quite different in each case.