R-matrix calculations for opacities: IV. Convergence, completeness, and comparison of relativistic R-matrix and distorted wave calculations for Fe xvii and Fe xviii

To investigate the completeness of coupled channel (CC) Breit–Pauli R-matrix (BPRM) calculations for opacities we employ the relativistic distorted wave (RDW) method to complement (‘top-up’) and compare the BPRM photoionization cross sections for high- n ℓ levels of both Fe xvii and Fe xviii . Good agreement is found with background photoionization cross sections using these two methods, which also ensures the correct matching of bound level cross sections for completeness. In order to top-up the CC-BPRM calculations, bound–bound transitions involving additional bound levels, and a large number of doubly-excited quasi-bound levels corresponding to BPRM autoionizing resonances described in the paper RMOPII are calculated using the RDW method. Photoionization cross sections in the high energy region are also computed and compared up to about 500 Ry, and contributions from higher core level excitations than BPRM are considered. The effect of configuration interaction is investigated, which plays a significant role in correctly reproducing some background cross sections. Due to the fact that the additional RDW levels correspond to high- n ℓ bound levels that are negligibly populated according to the Mihalas–Hummer–Däppen equation-of-state (paper I), the effect on opacities is expected to be small.