On the validity of the ICFT R-matrix method : Fe xiv

Del Zanna, G. and Badnell, N. R. and Fernandez-Menchero, L. and Liang, G. Y. and Mason, H. E. and Storey, P. J. (2015) On the validity of the ICFT R-matrix method : Fe xiv. Monthly Notices of the Royal Astronomical Society, 454. pp. 2909-2917. ISSN 0035-8711

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    Recently, Aggarwal & Keenan published a Dirac R-matrix (darc) calculation for the electron-impact excitation of Fe xiv. A 136-level configuration-interaction/close-coupling (CI/CC) expansion was adopted. Comparisons with earlier calculations, obtained by Liang et al. with the intermediate coupling frame transformation (ICFT) R-matrix method, showed significant discrepancies. One of the main differences was that the Liang et al. effective collision strengths were consistently larger. Aggarwal & Keenan suggested various possible causes for the differences. We discuss them in detail here. We have carried out an ICFT R-matrix calculation with the same 136-level CI/CC expansion adopted by Aggarwal & Keenan, and compared the results with theirs and with those of Liang et al., which employed a much larger CI/CC expansion. We find that the main differences arise because of the different CC and CI expansions, and not because of the use of the ICFT method, as suggested by Aggarwal & Keenan. The significant increase in the effective collision strengths obtained by Liang et al. is mainly due to the extra resonances that are present because of the larger target expansion.