Unveiling Controlling Factors of the S₀/S₁ Minimum Energy Conical Intersection: A Theoretical Study

The minimum energy conical intersection (MECI) geometries play an important role in photophysics and photochemistry. Although a number of MECI geometries can be identified using quantum chemical methods, their chemical interpretation remains unclear. In this study, a systematic analysis was performed on the MECIs between the singlet (S₀) and lowest singlet excited (S₁) states of organic molecules. The frozen orbital analysis (FZOA), which approximates the excited states with minimal main configurations, was adopted to analyze the excitation energy components at the S₀/S₁ MECI geometries as well as the S₀ and S₁ equilibrium geometries. At the S₀/S₁ MECI geometries, the HOMO-LUMO gaps decreased as expected but did not disappear. The remaining gaps were balanced with the HOMO-LUMO Coulomb integrals. Furthermore, we discovered that the HOMO-LUMO exchange integrals became approximately zero. On the basis of this fact, a systematic interpretation of the S₀/S₁ MECI geometries has been described.