Reaction rates are usually strongly dependent on pressure, temperature and reactant concentrations. In many cases, determining these rates experimentally is either impossible, or requires considerable time and expense. For this reason, methods and models of theoretical reaction kinetics are used in combination with quantum chemistry computations in order to determine the reaction rates of individual elementary reactions.
In a first step, reaction rate coefficients dependent on energy and angular momentum are calculated by applying the RRKM theory of Rice, Ramsberger, Kassel and Marcus, and/or the Statistical Adiabatic Channel Model (SACM theory).The required molecule-specific parameters (e.g. oscillation frequencies and rotation constants), as well as the properties of the transition states of the reaction pathway of interest, are taken from quantum chemistry calculations.
Finally the pressure- and temperature-dependent reaction rate coefficients and the branching ratios of multichannel reactions can be calculated by master equation analysis.