For WFT calculations of HfCO, correlation-consistent cc-pVXZ-PP 38 basis sets were chosen for Hf and aug-cc-pVXZ for C and O, where X = T, Q or 5. Specifically, only a quadrupleζ quality (X = Q) basis set was used for all MRCI analysis, but we also compared results from triple-ζ, quadruple-ζ, and quintuple-ζ quality sets for CCSD(T) calculations. The plain cc-pVTZ(- PP) basis set was also tested for all atoms of HfCO and exclusively employed for its isovalent HfCS, HfCSe, HfCTe, and HfCPo species to reduce the computational expense. In all cases, the inner 60 electrons of Hf (1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 4d 10 4f 14 ) were substituted with the Stuttgart relativistic pseudopotential (ECP60). HfCO, HfCS, and HfCSe WFT energetics were used to investigate density functional theory (DFT) errors associated with 23 density functional approximations (DFAs) that fall into six rungs of "Jacob's ladder" using the Psi4 43 package. The 23 DFAs corresponded to semi-local generalized gradient approximations (GGAs) (BLYP, BP86, and PBE), meta-GGAs (TPSS, SCAN, M06-L, and MN15-L), global GGA hybrids (B3LYP, B3P86, B3PW91, and PBE0), meta-GGA hybrids (TPSSh, SCAN0, M06, M06-2X, and MN15), range-separated hybrids (LRC-ωPBEh and ωB97X), and double hybrids (B2GP-BLYP and PBE0-DH, DSDBLYP-D3BJ, DSD-PBEB95-D3BJ, and DSD-PBEP86-D3BJ) functionals, as implemented in a recently introduced workflow. In this workflow, the density of a B3LYP calculation is converged first and used as the starting point for all other DFA calculations. Starting from CCSD(T) geometries, the results from these DFAs were used to evaluate the single-point De for HfCO, HfCS, and HfCSe. In the past we have applied def2-XZVP (X = T, Q) basis sets to study TM based systems and in this work the larger X = Q set was applied for all DFT calculations. The dissociation energy of CO from a periodic Hf(001) surface of hexagonal Hf was determined using Quantum-ESPRESSO (ESI Figure ). For all calculations, a norm-conserving pseudopotential with a kinetic energy cutoff of 70 Ry was employed. The Hf surface was approximated as a 3x3x1.5 slab and the CO molecule was placed vertically on top of an Hf atom.