  Sophisticated atmospheric retrieval algorithms, such as Nested Sampling,
explore large parameter spaces by iterating over millions of radiative transfer
(RT) calculations. Probability distribution functions for retrieved parameters
are highly sensitive to assumptions made within the RT forward model. One key
difference between RT models is the computation of the gaseous absorption
throughout the atmosphere. We compare two methods of calculating gaseous
absorption, cross-sections and correlated-$k$, by examining their resulting
spectra of a number of typical \ce{H2}-He dominated exoplanetary and brown
dwarf atmospheres. We also consider the effects of including \ce{H2}-He
pressure-broadening in some of these examples. We use NEMESIS to compute
forward models. Our $k$-tables are verified by comparison to ExoMol
cross-sections provided online and a line-by-line calculation. For test cases
with typical resolutions ($\Delta \nu = 1$cm$^{-1}$), we show that the
cross-section method overestimates the amount of absorption present in the
atmosphere and should be used with caution. For mixed-gas atmospheres the
morphology of the spectra changes, producing `ghost' features. The two methods
produce differences in flux of up to a few orders of magnitude. The addition of
pressure broadening of lines adds up to an additional order of magnitude change
in flux. These effects are more pronounced for brown dwarfs and secondary
eclipse geometries. We note that correlated-$k$ can produce similar results to
very high-resolution cross-sections, but is much less computationally
expensive. We conclude that inaccurate use of cross-sections and omission of
pressure broadening can be key sources of error in the modelling of brown dwarf
and exoplanet atmospheres.
