2026A&A...705A.158T

Molecular collisional data are essential for accurately modeling molecular abundances and excitation in the interstellar medium (ISM), yet remain scarce for many sulfur-bearing species. In this work, we calculate collisional rate coefficients for the near-prolate asymmetric top molecule HCSCN in collision with helium. A highly accurate three-dimensional potential energy surface (PES) was constructed at the CCSD(T)-F12a/aug-cc-pVTZ level of theory. Utilizing this PES, we performed coupled state calculations of inelastic cross sections for rotational transitions between the first 71 rotational levels with energies below 20 cm^–1 and collisional energies up to 500 cm^–1, from which we derived collisional rate coefficients up to 100 K. The calculated PES is highly anisotropic, exhibiting numerous minima with the deepest value being 53 cm^–1 at certain positions in and out of the HCSCN molecule plane. The collisional rates obtained for the b-type transitions are of the same magnitude as those of a-type transitions, with a slight inferiority. The rotational relaxation rate coefficients have a propensity rule that favors |ΔJ| = 1, |Δk_a| = |Δk_c| = 0 for a-type transitions, while ΔJ = 0, |ΔK_a| = |ΔK_c| = 1 for b-type transitions. These data provide important insights into sulfur chemistry in the ISM and will aid in more accurate modeling of molecular abundances in environments such as TMC-1.