Force multiplier as computed by evaluating the numerical sum over the discrete set of spectral lines in the triangle-marked green curve. Panel a shows the saturation level indicated with the dashed red curve, a power-law region with the dashed blue line, and the locatton of a transition eotnl witt the dashed black line. Panel b shows the actual fit of two numerical results, where the triangle-marked green line has the same temperature and density as in (a) and the square-marked blue line assumes a different set of temperature and density. The derived fit parameters for each case are indicated in the corresponding legend.