Global ab initio potential energy surfaces for both the ground ($\tilde X{^1}A^{\prime} $X̃1A′) and excited ($\tilde A^1 A^{\prime \prime} $Ã1A′′) electronic states of HNO and vibrational states of the Renner-Teller $\tilde A^1 A^{\prime \prime} $Ã1A′′–

The global potential energy surfaces for both the ground ($\tilde X{^1}A^{\prime} $X̃1A′) and excited ($\tilde A^1 A^{\prime \prime} $Ã1A′′) electronic states of the HNO molecule have been constructed by three-dimensional cubic spline interpolation of more than 17 000 ab initio points, which have been calculated at the internal contracted multi-reference configuration interaction level with the Davidson correction using an augmented correlation-consistent polarized valence quadruple zeta basis set. The low-lying vibrational energy levels for the two electronic states of HNO have also been calculated on our potential energy surfaces including the diagonal Renner-Teller terms. The calculated results have shown a good agreement with the experimental vibrational frequencies of HNO and its isotopomers.