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Phosphorous species are of great interest in interstellar chemistry since they are the basic blocks for building life here on Earth. Modelling the abundance and environment of recently detected PO |$^{+}$| under non-local thermodynamic equilibrium (LTE) requires rotational spectra of the molecule along with accurate collisional rates with the most abundant species, hydrogen and helium. A new 4D ab initio potential energy surface (PES) of PO |$^{+}$| – H |$_{2}$| collision is calculated using CCSD(T)/CBS(DTQ) methodology considering rigid rotor approximation. The region containing the minima of the PES is augmented using neural networks (NNs) model while very high potentials (⁠|$\gt 2500$| cm |$^{-1}$| ) and asymptotic region have been approximated using Slater and R |$^{-4}$| functions, respectively. The close coupling calculations have been performed using molscat software for both ortho and para -H |$_{2}$| . The rate coefficients have been reported for transitions |$j-j^{\prime }=$| |$1-0$|⁠ , |$2-1$|⁠ , |$3-2$|⁠ , and |$5-4$| through which PO |$^{+}$| has been experimentally detected in interstellar medium (ISM). The rate coefficients for even and odd transitions of PO |$^{+}$| with para -H |$_{2}$| are compared with that of helium and are found to be 1.1–2.0 times higher. For even transitions (|$\Delta j = 2$|), the ortho -H |$_{2}$| rates are 10 per cent higher than para -H |$_{2}$| rates. However, the trend reverses in the case of odd transitions (|$\Delta j = 1$|) when higher J transitions are considered at low temperatures. At higher temperatures, the ortho rates cross the para -H |$_{2}$| rates and become larger than the latter. The new rate coefficients with both ortho and para -H |$_{2}$| will enable accurate modelling of the PO |$^{+}$| abundance in the ISM under non-LTE conditions. [ABSTRACT FROM AUTHOR]

The effect of para -hydrogen (j p = 0) collisions on the rotational de-excitation transitions of molecule NCCP is investigated in this study. The scattering information is obtained by spherically averaging a four-dimensional potential energy surface (4DPES) over various H2 molecule orientations. The calculations used the CCSD(T)-F12a method and aug-cc-pVTZ basis set to generate a 4DPES for the NCCP–H2 van der Waals system. Within the NCCP– para -H2 4DPES, a minimum energy point of 191.82 cm−1 is attained at a distance of 3.6 Å from the centre of mass of H2 and NCCP. To compute cross-sectional data for NCCP interacting with para -H2 (j p = 0), close coupling calculations are employed, encompassing total energies up to 600 cm−1. The resulting rate coefficients [ |$k_{j{\rightarrow }j^{\prime }}(T$|⁠)] are calculated across a temperature range spanning from 5 K to 200 K. In accordance with propensity, even Δ j = −2 transitions are highly preferred. Comparatively, the derived |$k_{j{\rightarrow }j^{\prime }}(T$|⁠) for NCCP–H2 are determined to be 1.5–4.5 times of NCCP–He. This observation implies that relying on a scaling factor of 1.38 to extrapolate rate coefficients for H2 collisions from those of NCCP–He is not a reliable approach. [ABSTRACT FROM AUTHOR]

NCCP, a phosphorous species, is believed to have been discovered in the carbon-rich star IRC+10216. Understanding collisional properties, such as cross-sections (⁠|$\sigma _{j\rightarrow j^{\prime }}$|⁠) and rate coefficients (⁠|$k_{j\rightarrow j^{\prime }}$|⁠), is important for the reliable determination of molecular abundance. The non-reactive collisions between NCCP and He species at low temperatures are the subject of this study. Calculations are based on new ab initio potential energy surface (PES) of NCCP–He. The PES calculations are carried out at the CCSD(T)–F12a in conjunction with the aug-cc-pVTZ basis set. The PES is found to have a global minimum towards the N end with a well depth of −46.40 cm−1. The ab initio points are analytically fitted on to the Legendre polynomial relevant for quantum scattering. From this fitted PES, the integral inelastic rotational cross-sections of NCCP with He collisions are computed for total energies up to 550 cm−1, using the accurate close coupling approach of quantum mechanics. The resonances are observed at low total energies due to quasi-bound states of the NCCP−He complex. Rate coefficients are determined among the 19 lowest rotational levels of NCCP by thermally averaging the cross-sections at low temperatures. The de-excitation rate coefficients increase with decreasing Δ j. [ABSTRACT FROM AUTHOR]