Comprehensive \textsl{Ab Initio}~Calculations of \ce{CO2-H2} and \ce{CO2-He} Collisional Properties

TL;DR

This work delivers a first-principles framework for CO2–H2 and CO2–He collisional properties, combining CCSD(T)-level PES generation with close-coupling dynamics in YUMI to produce cross sections, rate coefficients, and pressure-broadening parameters across 100–800 K. The authors introduce rigorous PB/PS theory, provide detailed PES fits and a robust dynamical pipeline, and demonstrate agreement with experimental data after prudent scaling, achieving the ~10% precision required for JWST-era exoplanet atmospheres. A key contribution is the generation of ro-vibrationally-resolved broadening data with rotational coverage up to |m|=50, along with Padé-based extrapolations that enable inclusion in HITRAN/HITEMP. The results establish a comprehensive ab initio foundation for collisional properties of a polyatomic target and offer a benchmark for extending similar approaches to more complex systems in atmospheric and astrophysical contexts.

Abstract

We present comprehensive \textsl{ab initio} calculations of CO$_{\rm 2}$-H$_{\rm 2}$ and CO$_{\rm 2}$-He collisional properties from first principles, employing CCSD(T), potential calculations together with close-coupling dynamical scattering in the \YUMI~framework. We derive (in)elastic cross sections, rate coefficients, and pressure-broadening parameters -- incl., their rotational dependence up to $|m|=50$, and temperature dependence over the range of 100-800 K. We provide Padé fits for the broadening coefficients as a function of rotational quantum number, enabling extrapolation of the results and integration into spectroscopic databases, including HITRAN and HITEMP. The computed potentials for both CO$_{\rm 2}$-H$_{\rm 2}$ and CO$_{\rm 2}$-He have a sub-percent precision, and the dynamics-solving code YUMI ultimately yields the collisional parameters. Among these, the scaled pressure broadening experimental values meet the 10\% precision requirement for exoplanetary sciences with \textit{JWST}. This contrasts with the parameters available before the present calculations, which at higher temperatures (T$>$400 K) deviate as much as 5$\times$ from the desired precision requirement. All derivations and collisional properties are provided with this manuscript, establishing the first of such a comprehensive ab initio foundation for collisional systems with a target molecule having more than two atoms.

Figures (11)

• Figure 1: Schematics of the deployed framework for calculating collisional properties of CO2-He and CO2-H2. Various collisional properties of CO2 by H2 and He are estimated, which is useful for a wide range of applications, including exoplanetary retrievals. *The calculation of pressure shift is deferred to follow-up work.
• Figure 2: Details of the computational flow for the post-treatment following the dynamics calculation with Yumi.
• Figure 3: Elastic and inelastic rates $k(T)$ for ground-state CO$_2$ colliding with He and para-H$_2$. Elastic rates are roughly an order of magnitude larger than inelastic ones.
• Figure 4: Left: Broadening CO2-He at 296 K as a function of rotational quanta, comparison with experiment. The experimental values are taken from Hendaoui2025 and chen2021, while the Padé approximation are fitted to our scaled calculations. Right: Broadening CO2-para H2 at 296K as a function of rotational quanta, comparison with experiment. The experimental values are taken from hanson2014 and padmanabhan2014.
• Figure 5: Broadening of CO2-He as a function of temperature for two transitions (m=2,12) from two different methods for calculations and their corresponding fit in Single Power Law and Double Power Law. At both the low and high temperatures, SPL, plotted in solid line, tends to show large relative residuals ($>$10%), while DPL, plotted in dotted line, yields a sub-percent precise fit. Residuals, which show the relative difference between the fit values vs the calculated values, are only shown for the SPL for clarity.