Automatic construction of transition states and on-the-fly accurate kinetic calculations for reaction classes in automated mechanism generators

Abstract

A detailed combustion mechanism of hydrocarbon fuels usually contains a large number of reactions and need to have kinetic data assigned to them, where the rate constants are usually given by approximate estimation leading to inaccuracies. In this work, a method for construction transition state (TS) and an isodesmic reactions correction scheme for accurate and automatic calculation of rate constants have been designed. These methods are tested using the cyclization reactions of QOOH radicals and the concerted elimination reactions of RO2 radicals. The results show that the automatic TS construction method is effective to construct the initial guesses of the transition states, the isodesmic reactions corrected results are close to the CCSD(T) results and some of the rate constants are significantly different from the rate constants used in the automated mechanism generator, indicating that it is necessary and significant to perform on-the-fly accurate calculation of rate constants in an automated mechanism generator.