Reaction Path Concept in Theoretical Chemistry

The concept of the reaction pathway has played a pivotal role in theoretical chemistry. Under the Born–Oppenheimer approximation, elementary chemical processes are governed by the potential energy surface, where the intrinsic reaction coordinate (IRC) is defined as the steepest descent path connecting the reactant (minimum), transition state (first-order saddle point), and product (minimum). Dynamical effects associated with the IRC, such as vibrational excitation due to reaction-path curvature in coordinate space, IRC-jump, and instabilities arising from valley-ridge transition points, are crucial for understanding reaction mechanisms beyond static pictures. Recent advances in automated reaction path search methods have enabled the construction of reaction path networks based on quantum chemical calculations, while the development of the reaction space projector (ResPer) allows ab initio molecular dynamics (AIMD) trajectories to be analyzed within this network framework. In addition, the Natural Reaction Orbital (NRO) method provides valuable insights into the underlying electronic motion along the IRC, offering a powerful tool for visualizing and interpreting chemical transformation processes.