reactionpaths

In chemistry, a reaction path is a continuous sequence of molecular geometries on a potential energy surface that connects reactant configurations to product configurations. It provides a schematic route by which a chemical transformation may occur. When the path is chosen to follow the minimum energy route, it is called the minimum energy path (MEP). A related concept is the intrinsic reaction coordinate (IRC), a one-dimensional path defined by the gradient of the potential energy and masses that follows the steepest descent from the transition state to reactants and products.

Energy profiles along a reaction path typically feature a transition state at the highest point, representing an energy barrier. The path passes through a saddle point on the potential energy surface; the local curvature defines reaction coordinates. In simple reactions, a single path suffices; in more complex reactions, multiple competing paths or stepwise sequences exist, with intermediate minima and multiple transition states.

Computationally, several methods exist to locate reaction paths. The nudged elastic band (NEB) and string method compute the MEP on a discrete chain of images. The dimer method and related algorithms locate saddle points. The intrinsic reaction coordinate can be constructed by following the mass-weighted gradient from the TS; both IRC and MEP depend on the accuracy of the underlying potential energy surface.

Reaction paths aid in understanding mechanisms, estimating activation barriers and rate constants, and interpreting kinetic isotope effects. They are also used in catalysis and materials science. Limitations include dependence on the chosen level of theory, neglect of dynamic effects such as solvent and temperature, and possible multiple competing paths in real systems.