Fundamental Study of Density Functional Theory Applied to Triplet State Reactivity: Introduction of the TRIP50 Data Set

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.