Comprehensive Ab Initio Quantum Computations of CO$_{\rm 2}$-H$_{\rm 2}$ and CO$_{\rm 2}$-He Collisional Properties

Abstract

We present comprehensive \textsl{ab initio} fully quantum calculations of CO$_{\rm 2}$--H$_{\rm 2}$ and CO$_{\rm 2}$--He collisional properties. Our framework combines CCSD(T) potential-energy-surface calculations with close-coupling dynamical scattering in the \YUMI~framework to derive elastic and inelastic cross sections, rate coefficients, and pressure broadening parameters. We characterize the rotational dependence of the broadening coefficients up to $j=25$ for CO$_{\rm 2}$--H$_{\rm 2}$ and $j=40$ for CO$_{\rm 2}$--He, and their temperature dependence over 40--800 K. We also provide Pad\'e fits as a function of rotational quantum number, enabling extrapolation and integration into spectroscopic databases including HITRAN and HITEMP. The resulting pressure broadening coefficients reproduce available experimental measurements on an absolute scale, without empirical correction factors, and meet the $\sim$10\% precision requirement identified for \textit{JWST}-era exoplanet atmospheric studies. This represents a substantial improvement over previously available parameters, which at higher temperatures ($T>400$ K) can fall outside the desired precision by up to a factor of five. All derivations, computed collisional properties, and database-ready products are provided with this manuscript. Together, these results establish a comprehensive \textsl{ab initio}, parameter-free, fully quantum foundation for CO$_2$ collisional broadening by H$_2$ and He, while demonstrating the transformative potential of the ab-initio approach for next-generation spectroscopic needs across planetary atmospheres, combustion, health sciences, and fusion-plasma diagnostics.