Electron-impact excitation of Fe$^{2+}$: a comparison of intermediate
  coupling frame transformation, Breit-Pauli and Dirac R-matrix calculations
  Modeling the spectral emission of low-charge iron group ions enables the
diagnostic determination of the local physical conditions of many cool plasma
environments such as those found in H II regions, planetary nebulae, active
galactic nuclei etc. Electron-impact excitation drives the population of the
emitting levels and, hence, their emissivities. By carrying-out Breit-Pauli and
intermediate coupling frame transformation (ICFT) R-matrix calculations for the
electron-impact excitation of Fe$^{2+}$ which both use the exact same atomic
structure and the same close-coupling expansion, we demonstrate the validity of
the application of the powerful ICFT method to low-charge iron group ions. This
is in contradiction to the finding of Bautista et al. [Ap.J.Lett, 718, L189,
(2010)] who carried-out ICFT and Dirac R-matrix calculations for the same ion.
We discuss possible reasons.
