H, He-like recombination spectra – IV. Clarification and refinement of methodology for l-changing collisions

Precise spectral diagnostic modelling of H i and He ii recombination spectra can constrain theoretical models that describe many astrophysical environments. Simple analytic expressions are of interest for collisional l-changing rate coefficients that are used by large-scale population modelling codes. We review, clarify, and improve upon the modified Pengelly & Seaton formulae of Guzman et al. We show that the recent poor results for it shown by Vrinceanu et al. are due to their misinterpretation of its usage. We also detail efficient numerical algorithms which should enable the full quantum mechanical expression for such rate coefficients to be used much more routinely by modelling codes. We illustrate with some collisional-radiative population modelling for hydrogen.