Electron impact excitation of Be-like ions: a comparison of DARC and ICFT results

Emission lines of Be-like ions are frequently observed in astrophysical plasmas, and many are useful for density and temperature diagnostics. However, accurate atomic data for energy levels, radiative rates (A-values) and effective electron excitation collision strengths ($\mathrm{{\rm Y}}$) are required for reliable plasma modelling. In general it is reasonably straightforward to calculate energy levels and A- values to a high level of accuracy. By contrast, considerable effort is required to calculate $\mathrm{{\rm Y}}$, and hence it is not always possible to assess the accuracy of available data. Recently, two independent calculations (adopting the $R$-matrix method) but with different approaches (DARC and ICFT) have appeared for a range of Be-like ions. Therefore, in this work we compare the two sets of $\mathrm{{\rm Y}}$, highlight the large discrepancies for a significant number of transitions and suggest possible reasons for these.