Variation of the chemical diffusion coefficient as a function of the radial coordinate throughout the star (4 M_⊙, Z_ini = 0.014, log D₀ = 3, X_c = 0.66). Different types of mixing regions are indicated. The top panel corresponds to setting ζ = −0.5 in Eq. (1), the bottom panel to setting ζ = −1. The graphs in red show the corresponding profile of the Brunt–Väisälä frequency (N). The right column shows zoom-ins of the outermost five per cent of the radius.