Molecular properties and excited state van der Waals potentials in the NO A2σ+ + O2 Xσg- collision complex

We characterize NO A(2)sigma(+) + O-2 X-3 sigma(-)(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 sigma(-)(g), NO X-2 pi, and NO A(2)sigma(+). Specifically, we show that highly augmented basis sets are necessary to properly represent the NO A(2)sigma(+) 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 angstrom with a depth of at least 20 cm(-1) that we believe is caused by NO A(2)sigma(+) 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 angstrom, 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)sigma(+) and O-2 X-3 sigma(-)(g), as predicted by earlier experiments.