Impact of the beyond-Voigt effects and bath mixture composition on collision-perturbed spectra of HD at conditions relevant for giant planet atmospheres. Panel a: relative error of the Voigt-profile approximation as a function of pressure and temperature, shown as the relative difference between the Voigt and mHT profiles at profile maximum. The panels show, from top to bottom, the R(0), R(1), and R(2) lines. Panels b–d: simulations of the HD spectra (blue lines) at conditions relevant for the Neptune atmosphere (the perturbing bath is 80% H₂ and 20% He). The spectra are generated with the mHT profile using HAPI based on the DPL temperature parametrization. As a reference, we show the same lines for the cases of pure H₂ and pure He perturbers (see the red and black lines, respectively). The blue shadows show the same simulations as the blue lines but generated with the simple Voigt profile. Panels b–d correspond to points b–d shown in the temperature-pressure maps in panel a (the three selected points lie on the Neptune temperature-pressure line). The three cases illustrate three different line-shape regimes. The first one, (b), is the low-pressure case in which the lines are broadened mainly by the Doppler effect, and the pressure-induced collisional effects do not dominate the line shapes. The intermediate-pressure case, panel c, illustrates the extreme non-Voigt regime (the differences between the blue curves and blue shadows reach almost a factor of 2; see also the green ridge in the maps in the bottom panel). The third case, panel d, illustrates the high-pressure regime at which the HD lines are well described by a simple Voigt profile (the blue shadows almost overlap with the blue lines), but setting a proper composition of the perturber gas components plays an important role.