Resolution of the forbidden (J=0 → 0) excitation puzzle in Mg-like ions

We investigate the source of the discrepancy between R-matrix and distorted-wave (DW) collision strengths for J-J'= 0-0 transitions in Mg-like ions, for example 3s(2) S-1(0)-3p(2) S-1(0), as reported previously. We find it to be due to the neglect of coupling, for example via 3s3p P-1(1), as done by most DW codes. We have implemented an option to account for such coupling as a perturbation within the AUTOSTRUCTURE DW code. This removes the discrepancy of a factor similar to 10 and similar to 100 for Fe14+ and S4+, respectively, for such transitions. The neglect of coupling would have affected (to some degree) the atomic data for a few weak optically forbidden transitions in other isoelectronic sequences if they were calculated with DW codes such as FAC and HULLAC. In addition, we compare the Fe14+ line intensities predicted with the R-matrix collision strengths against observations of solar active regions and flares; they agree well. For Fe14+, we suggest that the best density diagnostic ratio is 327.0/321.8 angstrom.