Energy levels, transition frequencies and Einstein A coefficients were taken from CDMS and JPL. Collisional rate coefficients are from Dumouchel et al (2010) who have also made calculations for HNC. The set of rate coefficients was scaled by a factor 1.37 to represent collisions with H₂.

We also provide older collisional calculations from Green & Thaddeus (1974) and Green (unpublished data), extrapolated for the temperature range from 5 to 1200 K, and including energy levels up to J=29. for collisions with He. This dataset may still be useful to model high-J line observations of HCN. For details on the extrapolation see Schöier et al. (2005).

A third datafile where the hyperfine splitting (hfs) of the J=1 level is taken into account is also presented. In this file, the transition frequencies for the J=2-1 transition have been calculated with all the hyperfine levels in the J=2 collapsed into a single level.

• References
  Dumouchel, F., Faure, A., Lique, F. 2010, MNRAS 406, 2488
  Green, S., Thaddeus, P. 1974, ApJ, 191, 653
  Schöier, F.L, van der Tak, F.F.S., van Dishoeck, E.F., Black, J.H. 2005, A&A 432, 369