In theories describing @[email protected] it is usually assumed that there is a transition state of more positive molar Gibbs energy between the reactants and the products through which an assembly of atoms (initially composing the @[email protected] of the reactants) must pass on going from reactants to products in either direction. In the formalism of '@[email protected]' the transition state of an @[email protected] is that set of states (each characterized by its own geometry and energy) in which an assembly of atoms, when randomly placed there, would have an equal @[email protected] of forming the reactants or of forming the products of that @[email protected] The transition state is characterized by one and only one imaginary frequency. The assembly of atoms at the transition state has been called an @[email protected] (It is not a @[email protected] according to the definition in this Compendium.) It may be noted that the calculations of reaction rates by the transition state method and based on calculated @[email protected] refer to the potential energy maximum at the @[email protected], as this is the only point for which the requisite separability of transition state coordinates may be assumed. The ratio of the number of assemblies of atoms that pass through to the products to the number of those that reach the @[email protected] from the reactants can be less than unity, and this fraction is the '@[email protected]' \(\kappa \). (There are also reactions, such as the gas-phase @[email protected] of simple @[email protected], that do not require '@[email protected]' and which therefore do not involve a transition state.)