Top: surface term for the reference solar model (connected black dots) for the l = 0 frequencies as a function of observed frequency scaled by ν_max. The surface term for a sample of 100 of the best models is also shown for the Sun (gray), with the mean value highlighted by the dashed line. The blue connected squares show the empirical surface correction δν_n,l (Eq. (7)) based on the reference model. Lower: the differences between the observed and corrected model frequencies as a function of scaled frequency, with the solar observational errors overplotted in blue. The shaded gray areas represent the mean and standard deviation of q for the same 100 models shown in the top panel. The dotted vertical lines delimit the region used to calculate the quality metric Q.