An Evaluation of Modern Density Functionals for the Bond Dissociation Energies of Diatomic Transition Metal Complexes.

Two benchmark sets of dissociation energies of diatomic transition-metal complexes have been assembled from previously published data, with selected reference values reassessed or updated where necessary, and used to evaluate a variety of density functionals. The BDE41 set of 41 3d complexes based on both experimental data and phaseless auxiliary-field quantum Monte Carlo reference data is the statistically more robust of the two sets. A number of local hybrid functionals top the list with the smallest deviations, including the strong-correlation (sc) corrected scLH22t and scLH22ta, and the range-separated variant ωLH23tP, followed by the previous top-performing global hybrids B97 and B97-1. While global hybrids and local hybrids without sc-corrections tend to underbind on average, sc-corrections may shift this to moderate overbinding. Nitride and oxide complexes account for much of the underbinding. Functionals based on neural-network local mixing functions underbind more clearly also in other cases, confirming that for these very flexible constructions a lack of transition-metal training data is more harmful to transferability than for less flexible functionals with heuristic mixing functions. The smaller and somewhat less reliable HBDE17 set of 4d/5d complexes reinforces several of the observations but appears to disfavor large exact-exchange admixtures even more strongly. In this case carbide and oxide complexes provide the most pronounced challenges. Nevertheless, several functionals perform not much worse than the most sophisticated post-Hartree-Fock reference calculations currently available based on CCSD(T) with multi-reference corrections. These benchmark sets may also prove useful for training and evaluating new approximate methods, an application we intend to explore in future work.