The recent development of organic visible-light active photosensitizers has enabled the development of many novel triplet transformations, the mechanistic studies of which often rely on computation due to the short lifetime of the excited state intermediates. However, in contrast to studies of ground state reactivity, there has been little discussion of the best practices when using density functional theory to model triplet state reactions. Here, we report the first benchmark of density functionals on triplet reaction mechanisms. Barrier heights and thermodynamic values were computed for a set of 50 organic reactions using 45 functionals, with reference values obtained using high-level DLPNO-CCSD(T) calculations extrapolated to the complete basis set limit. In the course of this study, we observed a common tendency for triplet SCF calculations to converge non-Aufbau solutions, resulting in catastrophic predictions in both thermochemistry and activation energy barriers and leading to errors as high as 26.4 kcal/mol. Modifications to the initial SCF guess are proposed as a solution to such errors, enabling accurate comparison of functional performance. Range-separated hybrid functionals were found to consistently outperform their non-range-separated versions, while rungs below hybrid meta-GGA produce high errors compared to reference values. We recommend the best-performing single hybrid functionals ωM06, ωB97M, M06-2X, and M05-2X for their balance of high accuracy and computational efficiency.
Hughes et al. (Thu,) studied this question.