  When computing cross-sections from a line list, the result depends not only
on the line strength, but also the line shape, pressure-broadening parameters,
and line wing cut-off (i.e., the maximum distance calculated from each line
centre). Pressure-broadening can be described using the Lorentz lineshape, but
it is known to not represent the true absorption in the far wings. Both theory
and experiment have shown that far from the line centre, non-Lorentzian
behaviour controls the shape of the wings and the Lorentz lineshape fails to
accurately characterize the absorption, leading to an underestimation or
overestimation of the opacity continuum depending on the molecular species
involved. The line wing cut-off is an often overlooked parameter when
calculating absorption cross sections, but can have a significant effect on the
appearance of the spectrum since it dictates the extent of the line wing that
contributes to the calculation either side of every line centre. Therefore,
when used to analyse exoplanet and brown dwarf spectra, an inaccurate choice
for the line wing cut-off can result in errors in the opacity continuum, which
propagate into the modeled transit spectra, and ultimately impact/bias the
interpretation of observational spectra, and the derived composition and
thermal structure. Here, we examine the different methods commonly utilized to
calculate the wing cut-off and propose a standard practice procedure (i.e.,
absolute value of 25~cm$^{-1}$ for $P\leqslant$~200~bar and 100~cm$^{-1}$ for
$P >$ ~200~bar) to generate molecular opacities which will be used by the
open-access {\tt MAESTRO} (Molecules and Atoms in Exoplanet Science: Tools and
Resources for Opacities) database. The pressing need for new measurements and
theoretical studies of the far-wings is highlighted.

Earth and Planetary Astrophysics;Instrumentation and Methods for Astrophysics