Benchmark and dynamical characterization of the stationary points of reactive atom + alkane and S2 potential energy surfaces

We describe a composite ab initio approach to determine the best technically feasible relative energies of stationary points considering additive contributions of the CCSD(T)/complete-basis-set limit, core and post-CCSD(T) correlation, scalar relativistic and spin-orbit effects, and zero-point energy corrections. The importance and magnitude of the different energy terms are discussed using examples of atom/ion + molecule reactions, such as X + CH 4 /C 2 H 6 and X − + CH 3 Y/CH 3 CH 2 Cl [X, Y = F, Cl, Br, I, OH, etc. ]. We test the performance of various ab initio levels and recommend the modern explicitly-correlated CCSD(T)-F12 methods for potential energy surface (PES) developments. We show that the choice of the level of electronic structure theory may significantly affect the reaction dynamics and the CCSD(T)-F12/double-zeta PESs provide nearly converged cross sections. Trajectory orthogonal projection and an Eckart-transformation-based stationary-point assignment technique are proposed to provide dynamical characterization of the stationary points, thereby revealing front-side complex formation in S N 2 reactions and transition probabilities between different stationary-point regions. We review composite ab initio and dynamical methods and their applications to characterize stationary points of atom/ion + molecule reactions.