New rate coefficients for rotationally inelastic collisions of OCS with para- and ortho-H₂

Carbonyl sulfide OCS(X¹Σ⁺) is one of the most abundant sulfurated molecules detected in the gas phase toward star-forming regions and plays a major role in interstellar chemistry. Accurate collisional rate coefficients for its rotational excitation with the most abundant colliding species (H₂) in the ISM are crucial for modeling OCS lines. In this work, we calculate a four-dimensional potential energy surface (4D-PES) of OCS interacting with H₂ using the CCSD(T)-F12 method and the aug-cc-pVTZ basis sets. A test on the cross sections showed that considering a single rotational level of H₂ is sufficient. In the case of para-H₂ (j₂ = 0), we treated H₂ as a spherical body and the dynamic calculation is based on an average PES over five orientations of H₂, which greatly reduces the computational cost, whereas for ortho-H₂ (j₂ = 1), a 4D-PES was used. Inelastic cross sections were calculated using the close-coupling theory for collision energies up to 1000 cm^-1. From the Maxwell-Boltzmann distribution of kinetic energies, cross sections were averaged to generate the collisional rate coefficients below 200 K and j₁ ≤ 22 (18) for OCS-para-H₂ (-ortho-H₂). This work provides the first calculation of the rate coefficients of OCS-ortho-H₂, as well as the first calculation performed using the ab initio PES and quantum close-coupling approach for OCS-para-H₂. The rotational rate coefficients are compared with previous data obtained for OCS-H₂ by Green and Chapman in 1978 for j₁ ≤ 12 levels with the coupled state approach, using an interaction potential adapted from an electron gas model for OCS-He. The agreement between the two results is discussed. The present new data will help to accurately model the OCS emission lines toward the young stellar objects.