Fine-structure splitting calculation in the Ar III ion: comparison of perturbative (Breit-Pauli) and non-perturbative (MCDF-EAL) predictions

We have calculated fine structure splitting in the Ar III ion using the R-matrix method. Two independent atomic structure calculations have been performed. Results from the Breit-Pauli – and the Dirac-Atomic – R-matrix relativistic calculations are analysed comparatively. Cross sections and collision strengths are provided for selected weak and intercombination transitions, allowing explicitly for resonance effects. Convergence of the partial wave expansion is ensured by examining the partial collision strengths at collision energies up to 20 Ry. Radiative data are also reported here. We discuss all these results and compare them, when possible. For the allowed transitions, for which the resonance structure is not dominant, the two calculations are nearly identical.