Light-Matter Coupling have shown a great promise to alter the photochemistry of molecules. Unlike traditional photochemistry, which uses light as an energy source, polariton chemistry uses quantized photons as active chemical catalysts to significantly change the shape of the potential energy surface in molecular systems, and thus, open up new possibilities to tune and control chemical reactions. Theoretical investigations play a vital role in understanding the fundamental limits and the basic principle of new chemical reactivities achieved by polariton chemistry. In particular, the molecule-cavity interactions have often been described as the following dipole gauge Hamiltonian