An intermediate-coupling R-matrix calculation of electron-impact excitation of Fe${\rm ^{4+}}$

For a number of years, there has been a major effort to calculate electron-impact excitation data for every ion stage of iron embodied by the ongoing efforts of the IRON project [1993 {\it Astron. Astrophys.} {\bf 279} 298]. Due to the complexity of the targets, calculations for the lower stages of ionization have been limited to either intermediate-coupling calculations within the ground-configurations or $LS$-coupling calculations of the ground and excited configurations. However, accurate excitation data between individual levels within both the ground and excited configurations of the low charge-state ions are urgently required for applications to both astrophysical and laboratory plasmas. Here we report on the results of the first intermediate-coupling $R$-matrix calculation of electron-impact excitation for Fe${\rm ^{4+}}$ for which the close-coupling (CC) expansion includes not only those levels of the 3d${\rm ^4}$ ground configuration, but also the levels of the {\rm 3d${\rm ^34s}$}, 3d${\rm ^34p}$, 3d${\rm ^34d}$ and ${\rm 3d^24s^2}$ excited configurations. With 359 levels in the CC expansion and over 2400 scattering channels for many of the ${\rm J\Pi}$ partial waves, this represents the largest electron-ion scattering calculation to date and it was performed on massively parallel computers using a recently developed set of relativistic parallel $R$-matrix programs.