In recent years, many efforts have been made to investigate the formation of the soot both by experiment and theory. Experimentally, high-resolution techniques such as mass spectroscopy, cavity ring-down spectroscopy and microwave spectroscopy are used to detect reaction products and intermediates in soot formation process . However, laboratory experiments can only probe the mechanisms under specific conditions due to limitations of the experimental techniques . While these experiments provide valuable information about the kinetics and thermodynamics of soot formation, they do not reveal atomic level details of PAH growth. Theoretical studies are needed to help provide a quantitative understanding of reaction mechanisms of soot formation that may ultimately guide the design of new technology to reduce soot generation.