Molecular properties and excited state van der Waals potentials in the NO A² Σ⁺ + O₂ XΣ_g⁻ collision complex

We characterize NO A2Σ⁺ + O₂ X³Σ_g⁻ van der Waals (vdW) Potential Energy Surface (PES) with RHF/RCCSD(T) and CASSCF/CASPT2 calculations. To do this, we first assess our computational setup to properly represent the individual molecular properties of O₂ X³Σ_g⁻, NO X2Π, and NO A²Σ⁺. Specifically, we show that highly augmented basis sets are necessary to properly represent the NO A₂Σ⁺ polarizability. Then, we optimize different vdW geometries, and provide BSSE corrected plots of the quartet vdW PES. The surfaces show a confined channel at a distance of approximately 6 Å with a depth of at least 20 cm⁻¹ that we believe is caused by NO A²Σ⁺ hyper-polarizability. At shorter distances, the channel is connected to a vdW basin centered around the O–N O–O linear geometry with an inter-molecular separation of 4.3 Å, and a depth of 95 cm⁻¹ at the RCCSD(T) level. A CASPT2 scan along the linear geometry show that this vdW basin exists on both the doublet and quartet excited surfaces. These results infer the existence of a collision complex between NO A²Σ⁺ and O₂ X³Σ_g⁻, as predicted by earlier experiments.