So what is the best DFT functional to use to calculate transition metal barrier heights?

A new database of transition metal reaction barrier heights (MOBH35) is presented. Benchmark energies (forward and reverse barriers and reaction energy) are calculated using DLPNO-CCSD(T), with a (T1) correction term, extrapolated to the complete basis set limit using a Weizmann-1-like scheme. Using these benchmark energies, the performance of a wide selection of density functional theory (DFT) exchange–correlation functionals, including the latest from the Martin, Truhlar, and Head-Gordon groups, is evaluated. It was found, using the def2-TZVPP basis set, that the ωB97M-V (MAD 1.7 kcal/mol), ωB97M-D3BJ (MAD 1.9 kcal/mol), PW6B95-D4 (MAD 2.1 kcal/mol), and revTPSS0-D4 (MAD 2.1 kcal/mol) hybrid functionals are recommended. The double-hybrid functionals revDOD-PBEP86-D4 (MAD 1.4 kcal/mol), B2K-PLYP (MAD 1.4 kcal/mol) and revDSD-PBEP86-D4 (MAD 1.5 kcal/mol) perform better, but this has to be balanced by their increased computational cost.

Want to know more? See Mark A. Iron and Trevor Janes, Journal of Physical Chemistry, 2019, 123, 3761-3781 and 6379-6380.