Theoretical Study of Relativistic Retardation Effects: the Abnormal Fine Structure of O II

Using multi-configuration Dirac–Fock and relativistic configuration interaction methods with high-order corrections, we report our precise calculation results of the fine-structure energy levels of the ground-state configuration of O II (1s22s22p3). Our calculated fine-structure splittings of 2D3/2,5/2 and 2P1/2,3/2 are abnormal. We elucidate that the transverse (Breit) interaction, i.e. relativistic retardation effect, plays an important role for the abnormal fine-structure splittings. Our calculation results are in good agreement with experimental measurements.