Revised study of the collisional excitation of HCO+ by H2(j = 0)

Abstract

Rotational excitation of the interstellar HCO+ by para-H2(j = 0) is investigated. The scattering calculations are based on a new potential energy surface (PES) for the HCO+–H2 van der Waals complex averaged over H2 orientations and considering both molecules as rigid rotors. The new ab initio PES was obtained from electronic structure calculations using a single and double excitation coupled cluster method with perturbative contributions from connected triple excitations [CCSD(T)]. The five atoms were described using the augmented correlation-consistent triple zeta basis set. For a better description of the van der Waals interaction, bond functions were placed approximately at mid-distance between the HCO+ and H2 centres of mass. Inelastic cross sections between rotational levels up to rotational momentum j = 20 were computed using a close coupling approach. Rate coefficients for temperatures ranging from 10 to 500 K were computed. The new rate coefficients were compared with previous results obtained for the HCO+ cation. Differences were found. We expect that the new rate coefficients will help to analyse present and future observations of this key ion for the interstellar chemistry.