Abstract

Context. An accurate analysis of the physical-chemical conditions in the regions of the interstellar medium in which C-3 is observed requires knowing the collisional rate coefficients of this molecule with He, H-2, electrons, and H. Aims. The main goals of this study are to present the first potential energy surface for the C-3 +H-2 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 C-3 with para- and ortho-H-2. 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 C-3 with H-2. The global minimum of the surface is found in the linear HH-CCC configuration. Rotational deexcitation state-to-state cross sections of C-3 by collision with para- and ortho-H-2 are computed. Furthermore, a reduced two-dimensional surface is developed by averaging the surface over the orientation of H-2. The cross sections for the collision with para-H-2 using this approximation and those from the four-dimensional PES agree excellently. Finally, a set of rotational rate coefficients for the collision of C-3 with para- and ortho-H-2 at low temperatures are reported.