Electron-impact excitation effective collision strengths (× n³) for the electric dipole transition 1s²¹S₀ − 1snp ¹P₁ of the ions Si^{12 +} and Fe^{24 +} versus the principal quantum number n around peak abundance temperature. ×: R-matrix results; ⋄: distorted wave results with basis set n = 1 − 8; °: distorted wave results with basis set n = 8 − 12; solid line: least-squares fit using points n = 2 − 5; dashed line: extrapolation using the last two points; dotted line: extrapolation using the last point; see text.