2 Calculations

In almost all our previous work referred to above, we have adopted the CIV3 program of Hibbert (1975) to perform our calculations for energy levels and radiative rates. However, in the cases of Ca XV and Fe XXI we also performed an additional set of calculations employing the GRASP code of Dyall et al. (1989), as relativistic effects are very important for these and other ions with similar or higher Z. The inclusion of CI in our GRASP calculations was restricted to that within the basic (above listed six) configurations only. On the other hand, the CIV3 calculations considered more elaborate CI including external orbitals, but included only one-body relativistic operators in a Breit-Pauli approximation, namely spin-orbit, spin-other-orbit, spin-spin, Darwin and mass correction terms. In the present work we find this approach of LSJ coupling scheme to be inadequate, as both CI and RE are equally important for the ions under consideration here, and therefore we have adopted the fully relativistic GRASP code (which is based on the jj coupling scheme), but with inclusion of larger CI. To be specific, CI has been included among the 1s²2s²2p², 2s2p³, 2p⁴, 2s²2p3$\ell$, 2s2p²3$\ell$, 2p³3$\ell$ and 2s²3$\ell{^2}$ configurations. These 15 configurations give rise to 251 fine-structure levels, but results for energy levels and radiative rates are reported for those among the lowest 46 levels of the first six configurations only. The Breit and QED corrections have been included in the calculations, and the option of EAL (extended average level) has been adopted, as in all our earlier works. In this type of calculation, a weighted trace of the Hamiltonian matrix is minimized, which yields a compromise set of orbitals describing closely lying states with moderate accuracy.