Ames-2021 CO2 Dipole Moment Surface and IR Line Lists: Toward 0.1% Uncertainty for CO2 IR Intensities

A highly accurate CO2 ab initio dipole moment surface (DMS), Ames-2021, is reported along with 12C16O2 infrared (IR) intensity comparisons approaching a 1–4‰ level of agreement and uncertainty. The Ames-2021 DMS was accurately fitted from CCSD­(T) finite-field dipoles computed with the aug-cc-pVXZ (X = T, Q, 5) basis for C atom and the d-aug-cc-pVXZ (X = T, Q, 5) basis for O atoms, and extrapolated to the one particle basis set limit. Fitting σrms is 3.8 × 10–7 au for 4443 geometries below 15 000 cm–1. The corresponding IR intensity, SAmes‑2021, are computed using the Ames-2 potential energy surface (PES), which is the best PES available for CO2. Compared to high accuracy IR studies for 2001i–00001 and 3001i–00001 bands, SAmes‑2021 matches NIST experiment-based intensities [SNIST‑HIT16 or SHIT20] to −1.0 ± 1.3‰, or matches DLR experiment-based intensities [SDLR‑HIT16/UCL/Ames] to 1.9 ± 3.7‰. This indicates the systematic deviations and uncertainties have been significantly reduced in SAmes‑2021. The SUCL2015 (or SHITRAN2016) have larger deviations (vs SDLR) and uncertainties (vs SDLR, SNIST) which are attributed to the less accurate Ames-1 PES adopted in UCL-296 line list calculation. The SAmes‑2021 intensity of 12C16O2 and 13C16O2 is utilized to derive new absolute 13C/12C ratios for Vienna PeeDee Belemnite (VPDB) with uncertainty reduced by 1/3 or 2/3. Further evaluation of SAmes‑2021 intensities are carried out on those CO2 bands discussed in the HITRAN2020 update paper. Consistent improvements and better accuracies are found in band-by-band analysis, except for those bands strongly affected by Coriolis couplings, or very weak bands measured with relatively larger experimental uncertainties. The Ames-2021 296 K IR line lists are generated for 13 CO2 isotopologues, with 18 000 cm–1 and S296 K > 1 × 10–31 cm/molecule cutoff and then combined with CDSD line positions (except 14C16O2). The Ames-2021 DMS and 296 K IR line lists represent a major improvement over previous CO2 theoretical IR intensity studies, including Ames-2016, UCL-296, and recent UCL DMS 2021 update. A real 1 permille level of agreement and uncertainty will definitely require both more accurate PES and more accurate DMS.