Relativistic MR-MP Energy Levels for K-like Fe

Relativistic Multi-Reference Moller-Plesset (MR-MP) many-body perturbation theory calculations are reported for the ground and excited states of Fe VIII. Energies are provided for states coming from the configurations 3sin3pn3d1)4P (1= s, p, d, and f) with m + n + p + q = 9, p 3, and q 1. The MR-MP results were compared with the recommended observed values in the CHIANTI-v8 database and with previous ab-initio calculations. The MR-MP calculations reproduce 90% of the available observed values within 3500 cm-1. The previous accurate abinitio calculations reproduce only 50% of the observed values within 3500 cm -1. The electronic structure of Fe VIII is very complex and our MR-MP results show that accurate calculations require the inclusion of not only 3s2-3d2, 3p2-3d2, and 3s3p-3d2 promotions but also of all electron correlations via perturbation theory.