Observed (black line) and synthetic (1D in red, 3D in blue) spectra for Arcturus and the Sun – all data smoothed with a Gaussian kernel to have a resolving power of about 11 500 as appropriate for the Gaia RVS. Both the normalized spectra and the residuals are shown for each star. A microturbulence of 1.5 km s^-1 was adopted for the 1D models, which were calculated with the LHD package.