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When computing cross-sections from a line list, the result depends not onlyon the line strength, but also the line shape, pressure-broadening parameters,and line wing cut-off (i.e., the maximum distance calculated from each linecentre). Pressure-broadening can be described using the Lorentz lineshape, butit is known to not represent the true absorption in the far wings. Both theoryand experiment have shown that far from the line centre, non-Lorentzianbehaviour controls the shape of the wings and the Lorentz lineshape fails toaccurately characterize the absorption, leading to an underestimation oroverestimation of the opacity continuum depending on the molecular speciesinvolved. The line wing cut-off is an often overlooked parameter whencalculating absorption cross sections, but can have a significant effect on theappearance of the spectrum since it dictates the extent of the line wing thatcontributes to the calculation either side of every line centre. Therefore,when used to analyse exoplanet and brown dwarf spectra, an inaccurate choicefor the line wing cut-off can result in errors in the opacity continuum, whichpropagate into the modeled transit spectra, and ultimately impact/bias theinterpretation of observational spectra, and the derived composition andthermal structure. Here, we examine the different methods commonly utilized tocalculate 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 theopen-access {\tt MAESTRO} (Molecules and Atoms in Exoplanet Science: Tools andResources for Opacities) database. The pressing need for new measurements andtheoretical studies of the far-wings is highlighted.

The impact of spectral line wing cut-off: Recommended standard method with application to MAESTRO opacity database