The internal partition functions (in logarithmic units) vs. lgT for ATLAS12/MHD-Q model of T_eff = 15   000 K and lgg = 4.0 (thick lines) in comparison with Kurucz’s data shown as thin lines. The panels show H I, He I and II, and the first three ionization stages of C, N, O, Ne and Si, respectively. The lowest right-hand panel shows the same, but for Fe III–V. The ionization states of I–V are plotted by means of lines as explained in the right-hand side of the figure.