The kinetic model is important to explore the combustion chemistry. It improves the understanding of ignition, low-temperature chemistry, pollutant formation, etc. in combustions.  Accurate detailed chemical kinetic models are also critical for high fidelity computational fluid dynamic simulations, which potentially guide the design of the next-generation low-emission combustion devices.

Recent advances in chemical kinetic model development have transformed from a largely empirical to a highly theoretical science. As a result, theoretical chemistry is playing an increasingly significant role in combustion chemistry. The accurate prediction of the temperature and pressure dependence of gas-phase reactions requires state-of-the-art implementations of a variety of theoretical methods: ab initio electronic structure theory, transition state theory, classical trajectory simulations, and the master equation.

1) Ab initio electronic structure theory: the energy, geometry (moments of inertia), and vibrational frequencies of reactants, transition states, and products on the potential energy surface (PES) are determined by solving the Schrödinger equation with the electronic molecular Hamiltonian. Since the exact solution for the Schrödinger equation can only be obtained for the hydrogen atom. Approximations about the nature of the wavefunctions of the electrons and their interactions are sought. Density functional theory (DFT) is widely used in combustion applications and used to determine the PES and associated geometries and frequencies. Higher level ab initio methods are used to refine the energies of crucial points on the PES.

2) Transition state theory: determines the reaction rates of elementary chemical reactions via transition state, which is the saddle point of a PES.

3) Classical trajectory simulation: describes the collisional energy transfer relevant to unimolecular reaction kinetics. Typically, the pairwise approximation method is used to develop the collisional CxHy + M (He, Ne, Ar, Kr, etc.) PESs.

4) Master equation: solves the connection between the microscopic dynamics and the phenomenological rate coefficients using the chemically significant eigenstate approach for the multiwell and multichannel

                                 Example: Reactions of polyaromatic hydrocarbon (PAH) formation