Three sets of scaling parameters for Fe XXI. The black squares (set A) correspond to those listed in Table 2. The purple diamonds (set B) correspond to the scaling parameters used in Fernández-Menchero et al. (2016). The red stars (set C, see Sect. 4.1) correspond to the third set of scaling parameters which have a smaller deviation with respect to the default set. The level energies, A-values, and effective collision strengths shown in Table 3 are very sensitive to the scaling parameters of 3s and 3d.