Collisional excitation of NH by H2: Potential energy surface and scattering calculations

Abstract

Collisional data for the excitation of NH by H2 are key to accurately derive the NH abundance in astrophysical media. We present a new four-dimensional potential energy surface (PES) for the NH–H2 van der Waals complex. The ab initio calculations of the PES were carried out using the explicitly correlated partially spin-restricted coupled cluster method with single, double, and perturbative triple excitations [RCCSD(T)-F12a] with the augmented correlation-consistent polarized valence triple zeta basis set. The PES was represented by an angular expansion in terms of coupled spherical harmonics. The global minimum corresponds to the linear structure with a well depth D e = 149.10 cm−1. The calculated dissociation energy D0 is found to be 30.55 and 22.11 cm−1 for ortho-H2 and para-H2 complexes, respectively. These results are in agreement with the experimental values. Then, we perform quantum close-coupling calculations of the fine structure resolved excitation cross sections of NH induced by collisions with ortho-H2 and para-H2 for collisional energies up to 500 cm−1. We find strong differences between collisions induced by ortho-H2 and para-H2. Propensity rules are discussed. The cross sections are larger for fine structure conserving transitions than for fine structure changing ones, as predicted by theory. These new results should help in interpreting NH interstellar spectra and better constrain the abundance of NH in interstellar molecular clouds.

Low-temperature reaction dynamics of paramagnetic species in the gas phase

“…While the reaction is endothermic—with a negligible rate coefficient under interstellar conditions—understanding the inelastic collision dynamics is important for modelling the abundance of NH in the ISM and interpreting astronomical NH spectra. Potential energy surfaces have been calculated for the NH–H 2 system, 114 with the global minimum corresponding to a van der Waals complex with a linear configuration, with the N end of NH located beside the H 2 moiety (in agreement with an earlier study). 115 Excitation cross sections (with total energies up to 500 cm −1 ) were also calculated for NH with ortho - and para -H 2 .…”

Molecular properties and excited state van der Waals potentials in the NO A2Σ+ + O2 XΣg− collision complex

“…This also provide another assessment of the reliability of our calculations, despite the importance of the BSSE in our results. Additionally, we computed a scan along the linear geometry at the SA(2)-CASSCF (19,13)/CASPT2 level in the C 2v point group to isolate the NO A 2 S + + O 2 X 3 S g À doublet and quartet states only.…”

“…To study vdW interactions, the Coupled Cluster (CC) method, in association with large augmented correlationconsistent basis sets is the method of choice for small to medium sized systems. [15][16][17][18][19][20] In addition, these studies highlight the importance of accounting for basis set superposition error (BSSE). When the distance between two molecules decreases, each of them will increasingly use the other molecule's basis functions to improve the description of its own wave-function.…”

An accurate set of H3O+ − H2 collisional rate coefficients for non-LTE modelling of warm interstellar clouds

“…Since there is no experimental observables for the H 3 O + − H 2 system, we cannot assess the accuracy of our interaction potential. However, from similar PES calculations by Faure et al (2016); Pirlot et al (2021); Godard Palluet et al (2022), where the accuracy of the potential was estimated based on a comparison with experimental dissociation and/or bound states energies, we expect that its overall accuracy is at the wavenumber level of accuracy and is always better than 10 per cents. For full details about the interaction potential and the analytical fit, see Paper I.…”