The reaction mechanism may consist of any number of chemical reactions involving the species named in the species data. A reaction may be reversible or irreversible; it may be a three-body reaction with an arbitrary third body and/or enhanced third-body efficiencies; it may have a Lindemann [5], Troe [6] or SRI [7] pressure fall-off formulation; it may have an arbitrary pressure-dependence defined by special fits or interpolation; it may involve a photon; and it may depend on a species temperature other than that of the bulk gas. Pressure-dependent Reactions of the Chemkin Theory Manual provides more detailed discussion of these different formulations. In this section, we describe the rules and syntax needed to enter different types of reactions in the Gas-phase Kinetics input file. The keywords for controlling these reaction parameters are described in Alphabetical Listing of Project Input Keywords .

Reaction data must start with a line that contains the word REACTIONS (or REAC). The lines following the REACTIONS line contain reaction descriptions, together with their Arrhenius rate coefficients. The reaction description is composed of a reaction path, reaction rate coefficients, and (optionally) some auxiliary information or keywords.