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 (CO2)-C-12-O-16 infrared (IR) intensity comparisons approaching a 1-4 parts per thousand 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 a sigma(rms), is 3.8 X 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 S-HIT20] to -1.0 +/- 1.3 parts per thousand, or matches DLR experiment-based intensities [SDLR-HIT16/UCL/Ames] to 1.9 +/- 3.7 parts per thousand. This indicates the systematic deviations and uncertainties have been significantly reduced in SAmes-2021. The S-UCL2015 (or S-HITRAN2016) have larger deviations (vs S-DLR) and uncertainties (vs S-DLR, S-NIST) which are attributed to the less accurate Ames-1 PES adopted in UCL-296 line list calculation. The SAmes-2021 intensity of (CO2)-C-12-O-16 and (CO2)-C-13-O-16 is utilized to derive new absolute C-13/C-12 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 S-296 K > 1 X 10(-31) cm/molecule cutoff and then combined with CDSD line positions (except (CO2)-C-14-O-16) . The Ames-2021 DMS and 296 K IR line lists represent a major improvement over previous CO2 theoretical IR intensity studies, including Ames2016, 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.

Automated summary

A highly accurate CO2 ab initio dipole moment surface (DMS), Ames-2021, is reported along with (CO2)-C-12-O-16 infrared (IR) intensity comparisons approaching a 1-4 parts per thousand level of agreement and uncertainty. This study indicates that the systematic deviations and uncertainties have been significantly reduced in SAmes-2021, and new C-13/C-12 ratios for Vienna PeeDee Belemnite (VPDB) have been derived using the SAmes-2021 intensity.