Rotational excitation of protonated carbon dioxide (HOCO⁺) in collisions with molecular hydrogen.

This article focuses on the calculation of the potential energy surface (PES) and collisional rate coefficients for rotational transitions of the protonated carbon dioxide ion (HOCO⁺) induced by collisions with molecular hydrogen (H₂) in the interstellar medium. Using explicitly correlated coupled cluster theory [CCSD(T)-f12a] and a correlation-consistent aug-cc-pVTZ basis set, the PES for the HOCO⁺–H₂ complex was computed, revealing a well depth of 2423 cm⁻¹ at an equilibrium separation of 6.21 a₀. Time-independent quantum scattering close coupling calculations based on this PES yielded state-to-state integral cross sections and rate coefficients for collisions with both para-H₂ and ortho-H₂, showing similar magnitudes for both nuclear spin modifications. The resulting rate coefficients, significantly larger than previously used scaled HOCO⁺–He data, have been prepared for inclusion in non-LTE radiative transfer models to improve the interpretation of astronomical observations of HOCO⁺ in various interstellar environments.