Reaction coordinate

A parameter or set of parameters that tracks progress along a chemical reaction pathway, typically from reactants through a transition state to products.

Potential energy surface: A function assigning potential energy to each possible arrangement of atoms in a molecular system. A reaction coordinate traces a path across this multidimensional energy landscape.
Molecular geometry: The three-dimensional arrangement of atoms in a molecule, specified by coordinates or internal distances and angles. Geometrical coordinates provide the variables from which many reaction coordinates are built.
Transition state theory: A theory that estimates reaction rates from the properties of configurations at a dividing surface near a transition state. Its dividing surface is commonly placed at a selected value of a reaction coordinate.
Reaction coordinate (physics): A generalized coordinate used to describe the progress of a system between states in physical models. The chemical meaning specializes a broader physical idea to molecular reactions.
Transition state: A high-energy molecular configuration at a saddle point on a reaction pathway. The coordinate often identifies the configuration separating reactants from products.
Bond order: A numerical description of the number and strength of chemical bonds between two atoms. Changing bond orders can track bond breaking and formation along a reaction.
Committor function: The probability that a system starting in a given configuration reaches products before reactants. It tests whether a proposed coordinate distinguishes reactant-like from product-like configurations.
Reaction extent: A stoichiometric measure of how far a chemical reaction has proceeded, defined from changes in species amounts. It measures bulk conversion rather than position along a molecular pathway.
Intrinsic reaction coordinate: The steepest-descent path from a transition state toward reactants and products on a potential energy surface. It defines a geometrical path linking a transition state to both sides of a reaction.
Collective variable: A function of microscopic system coordinates used to summarize a process with fewer variables. A reaction coordinate is often chosen as a collective variable for chemical change.

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