An improved study of HCO+ and He system: interaction potential,
  collisional relaxation and pressure broadening
  In light of its ubiquitous presence in the interstellar gas, the chemistry
and reactivity of the HCO+ ion requires special attention. The availability of
up-to-date collisional data between this ion and the most abundant perturbing
species in the interstellar medium is a critical resource in order to derive
reliable values of its molecular abundance from astronomical observations. This
work intends to provide improved scattering parameters for the HCO+ and He
collisional system. We have tested the accuracy of explicitly correlated
coupled-cluster methods for mapping the short- and long-range multi-dimensional
potential energy surface of atom-ion systems. A validation of the methodology
employed for the calculation of the potential well has been obtained from the
comparison with experimentally derived bound-state spectroscopic parameters.
Finally, by solving the close-coupling scattering equations, we have derived
the pressure broadening and shift coefficients for the first six rotational
transitions of HCO+ as well as inelastic state-to-state transition rates up to
j = 5 in the 5-100 K temperature interval.
