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

Résumé : 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, \citet{Niraula2022} 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 Letter, we introduce an end-to-end solution from ab initio calculations to pressure broadening, and use the 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$_2$ presents a key absorption feature for exoplanet research (primarily driven by the observation of gas giants) at $\sim$4.3$\mu$m and yet severely lack opacity data. We compute elastic and inelastic cross-sections for the collision of {ortho-}H$_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. Our proof-of-concept introduces a computationally affordable way to compute full-dimensional interaction potentials and scattering quantum dynamics with a precision sufficient to reduce the model-limited biases originating from the pressure broadening and thus support instrument-limited science with JWST and future missions.