Modelling of molecular emission spectra from interstellar clouds requires the calculation of rate coefficients for (de‐)excitation by collisions with the most abundant species. We calculate rate coefficients for the rotational and hyperfine (de‐)excitation of the hydrogen cyanide (HCN) by collisions with H2 (j= 0), the most abundant collisional partner in cold molecular clouds. The scattering calculations are based on a new ab initio potential energy surface for the HCN–H2 collisional system, averaged over the H2 orientations. Close‐coupling calculations of pure rotational cross‐sections are performed for levels up to j= 10 and for total energies up to 1000 cm−1. The hyperfine cross‐sections are then obtained using a recoupling technique. The rotational and hyperfine cross‐sections are used to determine collisional rate coefficients for temperatures ranging from 5 to 100 K. A clear propensity rule in favour of even Δj rotational transitions is observed. The usual Δj=ΔF propensity rules are observed for the hyperfine transitions. The new rate coefficients are compared with the previous results obtained for the HCN molecule. Significant differences are found, mainly due to the use of H2 as a collisional partner instead of He. The new rate coefficients will significantly help in interpreting HCN emission lines observed with current and future telescopes.

The low-temperature rotational (de-) excitation of C 3 (X 1 AE þ g ) by collisions with He ( 1 S ) is studied using an ab initio potential energy surface (PES). This PES has been calculated using the single-and double-excitation coupledcluster approach with noniterative perturbational treatment of triple excitations [CCSD( T)] and the augmented correlation-consistent triple-basis set (aug-cc-pVTZ ) with bond functions. This PES is then incorporated in full close-coupling quantum scattering calculations for collision energies between 0.1 and 50 cm À1 in order to deduce the rate constants for rotational levels of C 3 up to j = 10, covering the temperature range 5Y15 K.

Modelling of molecular emission from interstellar clouds requires the calculation of rate coefficients for excitation by collisions with the most abundant species. From a new, highly correlated, two‐dimensional potential energy surface, rotational excitation of the C2H(X2Σ+) molecule by collision with He is investigated. State‐to‐state collisional excitation cross‐sections between the 25 first fine structure levels of C2H are calculated for energies up to 800 cm−1 which yields after thermal averaging rate coefficients up to T= 100 K. The exact spin splitting of the energy levels is taken into account. The recoupling technique introduced by Alexander & Dagdigian allows us to deduce the corresponding temperature‐dependent hyperfine state‐to‐state rate coefficients. Propensity rules are discussed.

For the van der Waals C(2)(X (1)Sigma(g)(+))-H(2) molecular system, we generated a new ab initio potential energy surface (PES). We mapped this PES at the multireference internally contracted configuration-interaction method including the Davidson correction together with a large diffuse basis set. Then, we incorporated our PES into quantum scattering calculations at the close coupling and infinite order sudden approximation methods to cover collision energies ranging from 0.1 up to 4000 cm(-1). After Boltzmann thermal averaging, rate coefficients for temperatures of up to 1000 K are deduced. Discrepancies between our new rates and those computed previously are noticed. This should induce deviations in astrophysical modeling.