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)/cc-pVTZ level.