Within Ansys Chemkin programs, information about a species (for example, a thermodynamic property) is presumed to be available in ordered arrays beginning with the first gas-phase species. When surface chemistry is included, the species array continues from the last gas-phase species, through the surface species, and ending with the last bulk species. In the expressions and equations in this manual, we presume that there are a total of species in the system. We use the index or subscript to refer to a specific species in the array. There are gas-phase species, which, by convention, are always the first entries in the species arrays. The index of the first gas-phase species is ( by our convention) and the last gas-phase species index is (). Thus, the gas-phase species indices are . In a similar way surface species indices are in the range and bulk species are in the range . The surface species may be arranged on any number of sites, and the bulk species may exist in any number of bulk mixtures. Furthermore, many situations occur where there are no surface species and/or no bulk species.

As discussed in Chemistry—Species and Phases , the species are grouped in "phases." The first (and often only) phase is the gas phase, whose index . When surface and/or bulk phases are considered, it is possible to have multiple surface site phases as well as multiple bulk (condensed) phases. In terms of indexing and nomenclature, the next phases after the gas phase will be the surface sites, whose phase indices are bounded by . The final phases are the bulk mixtures, whose indices are bounded by . In each phase there are species, and those species have indices in the range .

In most cases, users do not need to be concerned with the internal indexing of species within Ansys Chemkin. However, users who wish to create their own Chemkin-based programs will need to be aware of the use of species and phase arrays in passing information back and forth through the Chemkin/API. Also, these conventions are used extensively in the mathematical descriptions of our governing equations and rate formulations, so some familiarity is helpful in following the discussions here.