Potential energy surface and rovibrational energy levels of the H₂-CS van der Waals complex

Owing to its large dipole, astrophysicists use carbon monosulfide (CS) as a tracer of molecular gas in the interstellar medium, often in regions where H₂ is the most abundant collider. Predictions of the rovibrational energy levels of the weakly bound complex CS-H₂ (not yet observed) and also of rate coefficients for rotational transitions of CS in collision with H₂ should help to interpret the observed spectra. This paper deals with the first goal, i.e., the calculation of the rovibrational energy levels. A new four-dimensional intermolecular potential energy surface for the H₂-CS complex is presented. Ab initio potential energy calculations were carried out at the coupled-cluster level with single and double excitations and a perturbative treatment of triple excitations, using a quadruple-zeta basis set and midbond functions. The potential energy surface was obtained by an analytic fit of the ab initio data. The equilibrium structure of the H₂-CS complex is found to be linear with the carbon pointing toward H₂ at the intermolecular separation of 8.6 a_o. The corresponding well depth is −173 cm⁻¹. The potential was used to calculate the rovibrational energy levels of the para-H₂-CS and ortho-H₂-CS complexes. The present work provides the first theoretical predictions of these levels. The calculated dissociation energies are found to be 35.9 cm⁻¹ and 49.9 cm⁻¹, respectively, for the para and ortho complexes. The second virial coefficient for the H₂-CS pair has also been calculated for a large range of temperature. These results could be used to assign future experimental spectra and to check the accuracy of the potential energy surface.