Reaction kinetics are described in terms of reaction rate coefficients. The rate coefficients are evaluated in the transition state theory framework using information about the energies, geometries, and vibrational frequencies of reactants, products, and transition state. In order to achieve better agreement with experiment it might be advantageous to increase the reaction threshold value from the DFT calculated one and to scale calculated harmonic frequencies to account for DFT deficiency and for anharmonic effects.
The threshold correction can be applied either on a basis of more accurate ab initio calculations or based on the comparison of calculated and experimental rate coefficients. It appears that a value in the region of 6-7 kcal/mol describes DFT error adequately.
The recommended correction factors for DFT frequencies can be found at the NIST website for a variety of exchange-correlation functionals.
To calculate reaction kinetics
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Reaction kinetics theory
Analyzing CASTEP results
CASTEP Analysis dialog