Atomic data and spectral model for Fe III
  We present new atomic data (radiative transitions rates and collision
strengths) from large scale calculations and a non-LTE spectral model for Fe
III. This model is in very good agreement with observed astronomical emission
spectra, in contrast with previous models that yield large discrepancies with
observations. The present atomic computations employ a combination of atomic
physics methods, e.g. relativistic Hatree-Fock, the Thomas-Fermi-Dirac
potential, and Dirac-Fock computation of A-values and R-matrix with
intermediate coupling frame transformation and Dirac R-matrix. We study the
advantages and shortcomings of each method. It is found that the Dirac R-matrix
collision strengths yield excellent agreement with observations, much improved
over previously available models. By contrast, the transformation of
LS-coupling R-matrix fails to yield accurate effective collision strengths at
around 10^4 K, despite using very large configuration expansions, due to the
limited treatment of spin-orbit effects in the near threshold resonances of the
collision strengths. The present work demonstrates that accurate atomic data
for low ionization iron-peak species is now within reach.
