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There are major discrepancies between recent ICFT (Intermediate CouplingFrame Transformation) and DARC (Dirac Atomic R-matrix Code) calculations(Fern\'andez-Menchero et al. 2014, Astron. Astroph. 566 A104, Aggarwal et al.2016 Mon. Not. R Astr. Soc. 461 3997) regarding electron-impact excitationrates for transitions in several Be-like ions, as well as claims that the DARCcalculations are much more accurate and the ICFT results might even be wrong.To identify possible reasons for these discrepancies and to estimate theaccuracy of the various results, we carried out independent B-Spline R-Matrix(BSR) calculations for electron-impact excitation of the Be-like ion N$^{3+}$.Our close-coupling expansions contain the same target states (238 levelsoverall) as the previous ICFT and DARC calculations, but the representation ofthe target wave functions is completely different. We find close agreementamong all calculations for the strong transitions between low-lying states,whereas there remain serious discrepancies for the weak transitions as well asfor transitions to highly excited states. The differences in the final resultsfor the collision strengths are mainly due to differences in the structuredescription, specifically the inclusion of correlation effects, rather than thetreatment of relativistic effects or problems with the validity of the threemethods to describe the collision. Hence there is no indication that oneapproach is superior to another, until the convergence of both the targetconfiguration and the close-coupling expansions have been fully established.

journal of physics. b, atomic molecular and optical physics/journal of physics. b, atomic, molecular and optical physics

Electron-impact excitation of N$^{3+}$ using the B-spline R-matrix  method: Importance of the target structure description and the size of the  close-coupling expansion