Reliable prediction of the reaction barrier height with HF-DFT

Many systematic errors of semilocal DFT (GGA's and hybrids) are due to errors in the self-consistent density, and so can be reduced by using a more accurate density. For anions, radicals in solution, stretched bonds, reaction barrier heights, and even spin-crossover complexes, DFT results are substantially improved by using Hartree-Fock densities (called HF-DFT). We revisit the case of reaction barriers, showing that while HF-DFT can reduce barrier height errors by a factor of 2 or 3, even for hybrid functionals, different classes of barriers behave in different ways.