We have carried-out two intermediate coupling frame transformation ( ICFT ) R -matrix calculations for the electron-impact excitation of \mathrm { C } -like \mathrm { Fe } ^ { 20 + } , both of which use the same expansions for their configuration interaction ( CI ) and close-coupling ( CC ) representations .
The first expansion arises from the configurations \mathrm { 2 s ^ { 2 } 2 p ^ { 2 } , 2 s 2 p ^ { 3 } , 2 p ^ { 4 } } , \mathrm { \ { 2 s ^ { 2 } 2 p, 2 s 2 p ^ { 2 } , 2 p ^ { 3 } \ } } nl , with n = 3 , 4 , for l = 0 - 3 , which give rise to 564 CI/CC levels .
The second adds configurations \mathrm { 2 s ^ { 2 } 2 p 5 l } , for l = 0 - 2 , which give rise to 590 CI/CC levels in total .

Comparison of oscillator strengths and effective collision strengths from these two calculations demonstrates the lack of convergence in data for n = 4 from the smaller one .
Comparison of results for the 564 CI/CC level calculation with an earlier ICFT R -matrix calculation which used the exact same CI expansion but truncated the CC expansion to only 200 levels demonstrates the lack of convergence of the earlier data , particularly for n = 3 levels .
Also , we find that the results of our 590 CC R -matrix calculation are significantly and systematically larger than those of an earlier comparable Distorted Wave-plus-resonances calculation .

Thus , it is important still to take note of the ( lack of ) convergence in both atomic structural and collisional data , even in such a highly-charged ion as Fe ^ { 20 + } , and to treat resonances non-perturbatively .
This is of particular importance for Fe ions given their importance in the spectroscopic diagnostic modelling of astrophysical plasmas .