Topology of Potential Energy Surfaces near the X 2 A 1 /A 2 B 2 Conical Intersections in NO 2 : Coupled Cluster Calculations

The conical intersections (CoIns) between the ground state and the first excited state in NO₂ are investigated using single-reference coupled-cluster (CC) theory. The topology of potential energy surfaces (PESs) near the X²A₁/A²B₂ CoIns is characterized. Several critical points, including the minima of both states, the two transition states on the ground state, and the minimum energy of CoIn (MECI), are identified. The topology of the CoIn seam and the potential energy curves along the two branching space vectors in the vicinity of MECI are analyzed. Furthermore, based on the topological features of the PESs, two possible adiabatic and nonadiabatic evolution mechanisms following photoexcitation of NO₂ are discussed.