Ab initio quantum dynamics as a scalable solution to the exoplanet opacity challenge: A case study of CO$_2$ in hydrogen atmosphere

Light-matter interactions lie at the heart of our exploration of exoplanetary atmospheres. Interpreting data obtained by remote sensing is enabled by meticulous, time- and resource-consuming work aiming at deepening our understanding of such interactions (i.e., opacity models). Recently, Niraula et al. 2022 pointed out that due primarily to limitations on our modeling of broadening and far-wing behaviors, opacity models needed a timely update for exoplanet exploration in the JWST era, and thus argued for a scalable approach. In this proof-of-concept study, we introduce an end-to-end solution from \textsl{ab initio} calculations to pressure broadening, and use a perturbation framework to identify the need for precision to a level of $\sim$10\%. We focus on the CO$_2$-H$_2$ system as CO$_2$ is a key absorption feature for exoplanet research (primarily in many gas giants) at $\sim$4.3$\mu$m as pressure-broadening parameters required for interpreting such observations remain sparse. We compute elastic and inelastic cross-sections for the collision of {ortho-}H$_2$~with CO$_2$, in the ground vibrational state, and at the coupled-channel fully converged level. For scattering energies above $\sim$20~cm$^{-1}$, moderate precision inter-molecular potentials are indistinguishable from high precision ones in cross-sections. Our calculations agree with the currently available measurement within 7\%, i.e., well beyond the precision requirements.