An ab initio based full-dimensional potential energy surface for OH + O₂⇄ HO₃ and low-lying vibrational levels of HO₃

To provide an in-depth understanding of the HO₃ radical and its dissociation to OH + O₂, a six-dimensional potential energy surface (PES) has been constructed by fitting 2087 energy points for the electronic ground state of HO₃ (X²A'') using the permutation invariant polynomial-neural network (PIP-NN) approach. The energy points were calculated using an explicitly-correlated and Davidson-corrected multi-reference configuration interaction method with the correlation-consistent polarized valence double zeta basis (MRCI(Q)-F12/VDZ-F12). On the PES, the trans-HO₃ isomer is found to be the global minimum, 33.0 cm^-1 below the cis-HO₃ conformer, which is consistent with previous high-level theoretical investigations. The dissociation to the OH + O₂ asymptote from both conformers is shown to be barrierless. As a benchmark from a recently developed high-accuracy thermochemistry protocol, D₀ for trans-HO₃ is calculated to be 2.29 ± 0.36 kcal mol^-1, only slightly deeper than the value of 2.08 kcal mol^-1 obtained using the PES, and in reasonable agreement with the experimentally estimated value of 2.93 ± 0.07 kcal mol^-1. Using this PES, low-lying vibrational energy levels of HO₃ are determined using an exact quantum Hamiltonian and compared with available experimental results.