High-level ab initio studies of the complex formed between CO and O2

•The most stable complex of CO-O2 has slipped parallel structure.•Close in energy is a CO-O2 complex with perpendicular arrangement.•Another structure, with C2v symmetry and O2-OC alignment, is also close in energy.•The explicitly correlated coupled cluster method was used.

The explicitly correlated CCSD(T)-F12 method with VXZ-F12 basis sets was used to find the most stable structures of the van der Waals CO-O2 complexes. With geometry optimizations performed up to the quadruple-zeta level and basis set extrapolation, the calculated interaction energies for the triplet complexes are 123 cm−1 for the H complex in Cs symmetry (slipped near-parallel structure), 118 cm−1 for the X complex, also in Cs symmetry (perpendicular alignment) and 116 cm−1 for the CO-O2 T complex in C2v symmetry. The corresponding CCSD(T)-F12 results using the aug-cc-pVXZ basis sets are nearly the same. Similar calculations were performed for the CO-O2 singlet complexes, which are shown to have much higher stabilization energies, the highest being 206 cm−1 for the X complex.

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