Calculated 14N2 16O line intensities using Radau coordinates and an accurate potential energy surface

Abstract

Line intensities of the $14\text{N}2$ $16\text{O}$ molecule are calculated using the DVR3D nuclear-motion program suite. Recently, we (Mizus et al., JQSRT, 344, 109463 (2025)) presented a potential energy surface (PES) fitted to empirical energy levels with an accuracy close to 0.002 ${\text{cm}}^{-1}$ and very accurate dipole moment surfaces (DMS) calculated ab initio using a MRCI (multi-reference configuration interactions) level of theory. However, these accurate PES and DMS did not yield uniformly accurate calculated line intensities: some transition intensities disagreed with the measured ones by orders of magnitude. Here we analyze the reasons for these inaccurately calculated line intensities and develop a spectroscopic model which gives consistently accurate intensity predictions. This improvement is based on a relative minor improvement in the accuracy of the PES, the need for which was highlighted by changing the internal coordinates used in the calculation from Jacobi to Radau. In particular, the new model predicts line intensities with close to experimental accuracy for those vibrational bands, namely ($v_1{v}_2\ell v_3$) equals (5000), (4200), (3200) and (0112), whose intensities have been measured with sub-percent accuracy.