Approaching experimental accuracy for triatomic spectra using variational calculations: Potential energy and dipole moment surfaces of 14N216O

The lower accuracy for line positions obtained using variational calculations (VCs) compared to effective Hamiltonian approaches is the main drawback in the VC methodology. The ability of VCs to make predictions for all the bands up to dissociation, treat several isotopologues at once and to incorporate first principle ab initio methods makes VCs the preferred method of constructing molecular line lists which are now widely used in astrophysics, planetary science and metrology. A significant step bringing VC of 14N216O line positions close to experimental accuracy is reported. High accuracy semi-empirical PESs are constructed by fitting to experimentally-derived rovibrational energy levels starting from available ab initio PESs. Three fitted PESs are presented, the best fit reproduces 279 energy levels up to 7000 cm-1 with J = 0,2 and 5 with a standard deviation of 0.004 cm-1, which drops to 0.0028 cm-1 when about 10 % of these levels (29 levels to be precise) are removed from the fit. The extrapolation to higher J quantum numbers is shown to be very accurate up to J = 15. For extrapolation to higher J values, an additional fit of higher J energy levels is conducted, which results in a standard deviation of 0.005 cm-1 for 535 energy levels with J = 0, 2, 5,10, and 15. An ab initio dipole moment surface is computed and used to produce transition intensities in good agreement with the observations.