Modelling of molecular emission from interstellar clouds requires the calculation of rates for excitation by collisions with the most abundant species. This paper deals with the calculation of rate coefficients for rotational excitation of the HCN and HNC molecules in their ground vibrational state in collision with He. Calculations of pure rotational (de‐)excitation cross‐sections of HCN and HNC by He were performed using the essentially exact close‐coupling method. Cross‐sections for transitions among the 26 first rotational levels of HCN and HNC were calculated for energies up to 3500 cm−1. These cross‐sections were used to determine collisional rate constants for temperatures ranging from 5 to 500 K. The propensity rules of both collisional systems are discussed. A propensity for even Δj transitions is observed in the case of HCN–He collisions whereas a propensity for odd Δj transitions is observed in the case of HNC–He collisions. These propensities become less pronounced at high temperature, although they do not vanish within the temperature range considered in this work. The new rate coefficients will significantly help in interpreting HCN and HNC emission lines observed with current and future telescopes. In particular, the HNC/HCN abundance ratio derived from observations would have to be revised from values >1 to values ≤1.

The BASECOL2012 database is a repository of collisional data and a web service within the Virtual Atomic and Molecular Data Centre (VAMDC, http://www.vamdc.eu). It contains rate coefficients for the collisional excitation of rotational, ro-vibrational, vibrational, fine, and hyperfine levels of molecules by atoms, molecules, and electrons, as well as fine-structure excitation of some atoms that are relevant to interstellar and circumstellar astrophysical applications. Submissions of new published collisional rate coefficients sets are welcome, and they will be critically evaluated before inclusion in the database. In addition, BASECOL2012 provides spectroscopic data queried dynamically from various spectroscopic databases using the VAMDC technology. These spectroscopic data are conveniently matched to the in-house collisional excitation rate coefficients using the SPECTCOL sofware package (http:// vamdc.eu/software), and the combined sets of data can be downloaded from the BASECOL2012 website. As a partner of the VAMDC, BASECOL2012 is accessible from the general VAMDC portal (http://portal.vamdc.eu) and from user tools such as SPECTCOL.

We report extensive theoretical calculations on the rotation-inversion excitation of interstellar ammonia (NH 3 ) due to collisions with atomic and molecular hydrogen (both para-and ortho-H 2 ). Close-coupling calculations are performed for total energies in the range 1-2000 cm −1 and rotational cross sections are obtained for all transitions among the lowest 17 and 34 rotation-inversion levels of ortho-and para-NH 3 , respectively. Rate coefficients are deduced for kinetic temperatures up to 200 K. Propensity rules for the three colliding partners are discussed and we also compare the new results to previous calculations for the spherically symmetrical He and para-H 2 projectiles. Significant differences are found between the different sets of calculations. Finally, we test the impact of the new rate coefficients on the calibration of the ammonia thermometer. We find that the calibration curve is only weakly sensitive to the colliding partner and we confirm that the ammonia thermometer is robust.

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.