2022A&A...657A..55S

Context. An accurate analysis of the physical-chemical conditions in the regions of the interstellar medium in which C₃ is observed requires knowing the collisional rate coefficients of this molecule with He, H₂, electrons, and H.
Aims. The main goals of this study are to present the first potential energy surface for the C₃ +H₂ complex, to study the dynamics of the system, and to report a set of rate coefficients at low temperature for the lower rotational states of C₃ with para- and ortho-H₂.
Methods. A large grid of ab initio energies was computed at the explicitly correlated coupled-cluster with single-, double-, and perturbative triple-excitation level of theory, together with the augmented correlation-consistent quadruple zeta basis set (CCSD(T)-F12a/aug-cc-pVQZ). This grid of energies was fit to an analytical function. The potential energy surface was employed in close-coupling calculations at low collisional energies.
Results. We present a high-level four-dimensional potential energy surface (PES) for studying the collision of C₃ with H₂. The global minimum of the surface is found in the linear HH-CCC configuration. Rotational deexcitation state-to-state cross sections of C₃ by collision with para- and ortho-H₂ are computed. Furthermore, a reduced two-dimensional surface is developed by averaging the surface over the orientation of H₂. The cross sections for the collision with para-H₂ using this approximation and those from the four-dimensional PES agree excellently. Finally, a set of rotational rate coefficients for the collision of C₃ with para- and ortho-H₂ at low temperatures are reported.