Quantum scattering of SiS with H2: potential energy surface and rate coefficients at low temperature.

Rotational excitation of the interstellar species SiS with H2 is investigated. We present a new four dimensional potential energy surface for the SiS-H2 system. Both molecules were treated as rigid rotors. Potential was obtained from the electronic structure calculations using a single- and double-excitation coupled cluster method with perturbative contributions from connected triple excitations [CCSD(T)]. The four atoms were described using the aug-cc-pVTZ basis sets. Bond functions were placed at mid-distance between the SiS center of mass and the center of mass of H2 for a better description of the van der Waals interaction. Additionally, at seven characteristic geometries, we calculated perturbational components of the interaction energy using symmetry-adapted perturbation theory approach to explain the anisotropy of the potential energy surface. Coupled-state calculations of the inelastic integral cross sections of SiS in collisions with para-H2 and ortho-H2 were calculated at low energies. After Boltzmann thermal averaging, rate coefficients were obtained for temperatures ranging from 5 to 50 K. Significant differences exist between para- and ortho-H2 results. The strongest collision-induced rotational SiS transitions are the transitions with Deltaj=2 for collisions with para-H2 and the transitions with Deltaj=1 for collisions with ortho-H2.