Abstract The singlet oxygen as one of important reactive oxygen species (ROS) plays an important role in many fields, such as combustion, medicine, biology, and materials science. The introduction of electronic structure of singlet oxygen is an important issue in the class of Photochemistry and Quantum Chemistry. In this paper, the electronic structure of singlet oxygen will be studied by point group theory and high-level ab initio quantum chemical calculations, and the results suggest that the first excited state of O2 corresponds to the Δg state with an open-shell and a closed-shell components. This work will help students to understand the relationship between the electronic structure and the wavefunction for the oxygen molecule.
