Molecular properties and excited state van der Waals potentials in the NO A 2 Σ + + O 2 XΣ g - collision complex.

We characterize NO A 2 Σ + + O 2 X 3 Σ 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 O 2 X 3 Σ g - , NO X 2 Π, and NO A 2 Σ + . Specifically, we show that highly augmented basis sets are necessary to properly represent the NO A 2 Σ + 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 A 2 Σ + 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 A 2 Σ + and O 2 X 3 Σ g - , as predicted by earlier experiments.