A 0.002 cm−1-Accurate PES for 14N216O

High-accuracy potential energy surface (PES) and rovibrational energy levels are essential for computational IR line lists used in (exo)planetary atmospheric spectroscopic analysis and modeling. We present a new 14N216O PES refinement achieving 0.001–0.002 cm−1 statistical accuracy for Evib ≤ 7000 cm−1 and Jmax = 88–100, relative to complete experiment-based rovibrational energy levels in RITZ, MARVEL, HITRAN2020, and NOSL-296 datasets. Building upon the high-quality ab initio Comp-I PES, the resulting D2n (and D2nB) PES outperform the Ames B1b PES, the UCL TYM PES, and the UCL 2025 PES series in both energy-resolved and J-resolved comparisons, exhibiting the smallest mean residuals and scatter below Evib = 8000 cm−1, as well as the highest fractions of |δ| < 0.0010 cm−1 and |δ| < 0.0005 cm−1. Robust analysis identified only seven outliers among the UCL-2025 reference level set; all remaining levels are retained to ensure resilient statistics. The D2n PES also shows stable IR intensities with the G10K dipole moment surface and reasonably consistent isotopologue accuracy. Analysis of J-resolved σrms highlights the critical role of reference-dataset accuracy and internal consistency. We discuss factors enabling (sub-)0.002 cm−1 accuracy and prospects for extending similar accuracy to higher energies, additional isotopologues, and other molecules.