Under certain conditions, some reaction rate expressions depend on pressure as well as temperature. Gas-phase Kinetics provides for two kinds of such reactions: unimolecular/recombination fall-off reactions and chemically activated bimolecular reactions. Generally speaking, the rate for unimolecular/recombination fall-off reactions increases with increasing pressure, while the rate for chemically activated bimolecular reactions decreases with increasing pressure. In both cases, Gas-phase Kinetics makes available various expressions that blend smoothly between high- and low-pressure limiting rate expressions.
Note: Chemkin makes the selection of pressure-dependent rate formulation equation based only on whether the LOW or HIGH keyword is specified in the auxiliary input for the rate coefficients. When LOW is specified, the coefficients given on the reaction line are assumed to represent the high-pressure limit and the pressure-falloff formulation is used. When HIGH is specified, the coefficients given on the reaction line are assumed to represent the low-pressure limit and the chemically activated reaction formulation is used.