Comparison of effective collision strengths for Ar¹³⁺. Left: all excitations among the 15 levels of the n = 2 configurations at the temperature (T_e = 4.0 × 10⁶ K) of peak fractional abundance in ionization equilibrium. The “ × ” symbol corresponds to the 2s²2p   ²P_3/2 − 2p^3   2P_3/2 (1–15) excitation shown in the righthand panel, which is linked for the two different previous calculations. Double-horizontal lines correspond to agreement within 20%. Right: the effective collision strength of the 1–15 transition as a function of temperature (K). ZGP94 and LBL10 correspond to the Breit-Pauli R-matrix works of Zhang et al. (1994) and Ludlow et al. (2010), respectively.