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.