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5 Discussion

In the calculation of effective collision strengths for the Na-like ion Fe XVI we have examined in detail several effects that must be taken into account in order to ensure a reasonable level of accuracy. In particular the contribution of the high partial waves for both allowed and forbidden transitions has been found difficult to manage, as the top-up procedures can only be implemented at high values of the collisional J symmetry. In the present work we have adopted the values of $J^{\rm oa}_{\max}\!=\!J^{\rm na}_{\max}\!=\!40$, which are adequate for most transitions although some of the allowed ones within the $n\!=\!4$complex still presented inacccurate high-energy tails that required truncation. We conclude from detailed comparisons with previous work that, in spite of the accuracy that is easily reached in target representation, the level of accuracy of the resulting effective collision strengths is probably not better than 10% for $\Upsilon \gt 0.1$ and 20% for the smaller ones. This is due to effects that come into play at high degrees of ionization. The present dataset is arguably the most reliable to date since we have made an attempt to identify the sources of error, and then take care of them so as to maintain accuracy. We have thus gained sufficient experience to tackle some of the more difficult iron ions that are being considered as part of the IRON Project.

Acknowledgements

Part of the present work was carried out during visits by MEG and CM to the Observatoire de Paris, Meudon, France. The hospitality received is gratefully acknowledged. The visits were funded by the CNRS, IVIC, CONICIT, Fundación Polar, the Observatoire de Paris and the Ministère des Affaires Étrangères. Computations were carried out at the Ohio Supercomputer Center, Columbus, Ohio, U.S.A., and at CeCalCULA, Universidad de Los Andes, Mérida, Venezuela. This research has been supported by CONICIT under contract No. S1-95000521. The authors are grateful to Dr. H.E. Saraph for a critical reading of the manuscript.


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