Abstract: In this article, a Hückel-Hubbard (H-H) parametrization has been proposed.With the H-Hparameters obtained, the authors first calculated the potential energy surfaces for the ground states and low-lying exited states of nitrogen ylide [2,3] and hydrogen [1,3] sigmatropic rearrangements in the frame of H-Htheory, Based on the calculated potential energy surfaces, the possible mechanisms of these ground state and excited state reactionswere discussed.and valuable conclusions were obtained.

Key words: Hückel-Hubbard parametrization, Sigmatropic rearrangement, Photochemicalreaction, Nonadiabatic transition