1 Introduction

The present calculation is part of an international collaboration known as the IRON Project (Hummer et al. 1993, referred to as Paper I) to obtain accurate collision rates for fine-structure transitions. Here we calculate fine-structure collision strengths for transitions within the 3d² complex of Fe VII over a sufficiently wide and fine energy mesh, in order to be able to integrate over a Maxwellian distribution to obtain the effective collision strength, from which the excitation and de-excitation rate coefficients can easily be obtained (Paper I).

Earlier work on these collision strengths was by Norrington & Grant (1987) using the Dirac R-matrix program. This is normally the most accurate approach when the scattering electron energy is comparable to the target level splitting. However, their calculation did not contain resonances arising from excited states above the 3d² states, and such resonance structures affect the collison strength right down to the 3d² states. A controversial feature of their results is that in the impact energy range from the highest 3d² threshold to beyond 3 Rydbergs, the collision strength from the Dirac R-matrix model decreased monotonically with energy, in contrast to that from the distorted-wave (DW) model of Nussbaumer & Storey (1982) which increased. This difference was ascribed to channel coupling included the R-matrix method but not in DW; it is such differences we want to clarify.