# Collisional excitation of HCN by CO to refine the modeling of cometary comae

## TL;DR

This paper provides the first dataset of collisional excitation and de-excitation rate coefficients for HCN induced by CO in cometary atmospheres, covering 5-50 K, to improve modeling accuracy of molecular distributions.

## Contribution

It introduces a novel dataset of rate coefficients for HCN-CO collisions, derived using the SACM method validated against quantum calculations, enhancing cometary atmosphere models.

## Key findings

- First dataset of HCN-CO collisional rates for 5-50 K.
- Validated SACM approach against quantum calculations.
- Improves modeling of HCN distribution in cometary atmospheres.

## Abstract

We present the first dataset of collisional (de)-excitation rate coefficients of HCN induced by CO, one of the main perturbing gases in cometary atmospheres. The dataset spans the temperature range of 5-50 K. It includes both state-to-state rate coefficients involving the lowest ten and nine rotational levels of HCN and CO, respectively, and the so-called "thermalized" rate coefficients over the rotational population of CO at each kinetic temperature. The derivation of these coefficients exploited the good performance of the statistical adiabatic channel model (SACM) on top of an accurate interaction potential computed at the CCSD(T)-F12b/CBS level of theory. The reliability of the SACM approach was validated by comparison with full quantum calculations restricted at the lowest total angular momentum of the system. These results provide essential input to accurately model the distribution among the rotational energy levels and the abundance of HCN in cometary atmospheres, accounting for deviations from local thermodynamic equilibrium that typically occurs in such environments.