Early versions of Ansys Chemkin allowed only integer stoichiometric coefficients. This was based upon the assumption that kinetic mechanisms would deal with elementary chemical reactions, for which it makes little sense to talk about a fraction of a molecule participating as a product or reactant. However, in many real-world applications the elementary reactions are not known. Instead, the kinetics may only be summarized in terms of global expressions. Gas-phase Kinetics and Surface Kinetics therefore allow use of non-integer stoichiometric coefficients. Examples of reactions with such non-integer coefficients are

(3–11)

and

(3–12)

The rate-of-progress of a reaction is, by default, still evaluated via Equation 3–4 , with the coefficients and defined as real numbers instead of integers. The Gas-phase Kinetics and Surface Kinetics Pre-processors automatically allow real coefficients for reactions without requiring any special flags or keywords. An executable can call subroutine SKIRNU to find out which reactions were declared to the Pre-processor with real coefficients, and get arrays of the coefficients.