Ordinary collision strengths (Ω) for Ar XIII. Left panel: refers to the forbidden transition from the ground level 2s²²p² (³P₀) to the metastable level 2s²²p² (³P₁), while the right panel refers to the transition from the ground level 2s²²p² (³P₀) – 2s2p³ (³D₁). The present work (PW) is shown in pink solid lines. The solid squares and the dashed lines are previous distorted wave approximations from Table 4 of Dere et al. (1979, D79).