NCCP collisions with para-H2: Accurate potential energy surface and quantum dynamics at interstellar temperatures

Abstract

ABSTRACT The effect of para-hydrogen (jp = 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 (jp = 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 Δj = −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.