  Precise spectral diagnostic modelling of H~{\sc i} and He~{\sc ii}
recombination spectra can constrain theoretical models which 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 Guzm\'an \etal We show that the recent poor
results for it shown by Vrinceanu \etal 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.
