Combining Multiconfiguration and Perturbation Methods: Perturbative Estimates of Core-Core Electron Correlation Contributions to Excitation Energies in Mg-Like Iron

Large configuration interaction (CI) calculations can be performed if part of the interaction is treated perturbatively. To evaluate the combined CI and perturbative method, we compute excitation energies for the states in Mg-like iron. Starting from a CI calculation including valence and core-valence correlation effects, it is found that the perturbative inclusion of core-core electron correlation halves the mean relative differences between calculated and observed excitation energies. The effect of the core-core electron correlation is largest for the more excited states. The final relative differences between calculated and observed excitation energies is 0.023%, which is small enough for the calculated energies to be of direct use in line identifications in astrophysical and laboratory spectra.