Sensitivity to collisional data in stellar model T_eff/log g/[Fe /H ] = 4500/1.0/−1.0. Left: abundance corrections ΔA(Mg) for the 4571 Å (upper) and 8806 Å (lower) lines. The solid lines and dots are ΔA(Mg) calculated with the model atom F described in Paper I. Broken lines are calculations done with modified collisional rates C → C × 2.0 (black) and C → C × 0.5 (grey) from the CE (dash), CH (dot-dash) or CH0 (three-dot-dash) rates used in the F model atom and derived from recent calculations (see text). Right: departure coefficients at A(Mg) = 6.6 of the levels involved in the 8806 Å (3d ¹D−3p ¹P) and the 4571(3p ³P−3s ¹S) transitions, together with the ground level of Mg ii, as function of optical depth. 3p ¹P has a very similar behaviour to 3p ³P so it is not shown in the figure to aid visualisation. Solid and broken lines have the same description as in the left figures; thick broken lines are for C × 2.0 and thin broken lines for C × 0.5 rate coefficients.