Photo-ionisation cross-section data used in the MULTI (red lines) and TLUSTY (black symbols) calculations for two b-f transitions of Mg I and the one from the ground level of Mg II. The red lines are the values taken from the TOPBASE database without the cross sections below 911 Å (except for the Mg II b-f transition). The symbols are the resonance-averaged photo-ionisation cross sections of the TOPBASE data (see Allende Prieto et al. 2003). The b-f cross section of the Mg II 3s(2S) and the Mg I 3p(3P) levels are vertically shifted by 4 and 2 log units, respectively, for visualisation.