The gas-phase kinetics input file, lists the chemical reactions and rate parameters for the system of interest. Gas-phase chemistry can often be described in terms of a series of elementary chemical reaction steps, because there is often a large body of scientific literature and methods to provide the needed data.

For some systems, particularly in the combustion field, it is often possible to assemble a reaction mechanism from parts of previously developed mechanisms. In such an assembly process, it is important to pay attention to differences in naming conventions used by various researchers and make sure a given species actually is the desired species. Matching thermochemical properties is important in determining species identification, as well as matching up reaction types with comparable rates. The Ansys Chemkin Mechanism Analyzer can be very useful in this process. A caution is that the reaction set included for a given chemical species in established reaction mechanism A may not be sufficient for simulations for purpose B. For example, if mechanism A includes a species as a minor product, it may not have the full complement of reactions necessary to correctly treat that species as a major fuel component in a different set of simulations. So the range of reaction types that are included for a given species should be checked while assembling a reaction mechanism from parts of other mechanisms.

In addition to published reaction mechanisms, there are a number of standard sources of chemical kinetics data for individual reactions. The Chemical Mechanism Data page includes a link to one of the best-known online resources: the NIST kinetics database [21]. This is an evaluated compilation of chemical kinetics data, and replaces earlier hard-copy compilations such as that by Kondratiev [22]. A search of the scientific literature may also lead to kinetic data that are not in the standard compilations.

If the desired kinetic data are not available in the literature for a species of interest, such data can often be calculated or estimated using standard techniques. For smaller molecules, quantum chemistry calculations are a viable, though computationally intensive, approach to obtaining kinetic data. Finding a transition state, needed to obtain activation energies, is generally more time-consuming than characterizing a stable species. Thus, calculations of kinetic parameters should be performed with more caution and care than calculations of thermochemical data. Computational results for the energetics of saddle points are often less certain than for minima in the potential-energy surface, such that kinetic parameters may need to be considered as having larger error bars than the corresponding species thermochemistry.

Benson's group additivity methods [20] offer a good way of estimating kinetic parameters as well as thermochemical data, especially for molecules composed of light elements. Simple analogies with related species also provide a method for estimating kinetic parameters, but such values should be expected to have substantial uncertainties.

Plasma simulations introduce additional complexities to the gas-phase chemistry in the form of electron-induced reactions, multiple electronically or vibrationally excited states, and the presence of charged species. The specialized subject of developing reaction mechanisms for plasma simulations has been summarized by Meeks and Ho [[23], [24]].