  We have carried-out two intermediate coupling frame transformation (ICFT)
R-matrix calculations for the electron-impact excitation of C-like 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 2s$^2$ 2p$^2$, 2s 2p$^3$, 2p$^4$, {2s$^2$ 2p, 2s 2p$^2$, 2p$^3$}
nl, with n=3,4, for l=0-3, which give rise to 564 CI/CC levels. The second adds
configurations 2s$^2$ 2p 5l, 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.
