Strong collision rate coefficients in unimolecular reaction rate theory

The traditional and a recently proposed renormalized approximation to the rate coefficient obtained from a strong collision master equation are derived and compared. It is shown that they deviate significantly when applied to the unimolecular reactions of large molecules with low activation energies at elevated temperatures. The derivation is extended to yield a method whereby the exact rate coefficient can be calculated without increasing the level of numerical effort. An analysis of the exact rate coefficient in the high and low collision frequency limits is found to verify the validity of the traditional approximate rate coefficient in these limits. The renormalized rate coefficient fails in the low frequency limit. Both approximations can be significantly in error at intermediate collision frequencies and the use of the exact expression is recommended.