Radical Addition Reactions: Hierarchical Ab initio Benchmark and DFT Performance Study

We performed a hierarchical ab initio benchmark study of the gas‐phase radical addition reactions of X• + C2H2 and X• + C2H4 (X•= CH3•, NH2•, OH•, SH•). The hierarchical series of ab initio methods (HF, MP2, CCSD, CCSD(T)) were paired with a hierarchal series of Dunning basis sets with and without diffuse functions ((aug)–cc–pVDZ, (aug)–cc–pVTZ, (aug)–cc–pVQZ). The HF ground‐state wavefunctions were transformed into quasi‐restricted orbital (QRO) reference wavefunctions to address spin contamination. Following extrapolation to the CBS limit, the energies from our highest‐ QRO–CCSD(T)/CBS+ level converged within 0.0–3.4 kcal mol–1 and 0.0–1.0 kcal mol–1 concerning the ab initio method and basis set, respectively. Our QRO–CCSD(T)/CBS+ reference data was used to evaluate the performance of 98 density functional theory (DFT) approximations. The MAE of the best functionals for reaction barriers and energies were: OLYP (1.9 kcal mol–1), BMK (1.0 kcal mol–1), M06–2X (0.9 kcal mol–1), MN12–SX (0.8 kcal mol–1) and CAM–B3LYP (0.8 kcal mol–1). These functionals also accurately reproduce key geometrical parameters of the stationary points within an average 2% deviation from the reference QRO–CCSD(T)/CBS+ level.