MANY-BODY PERTURBATION-THEORY IN ATOMIC-STRUCTURE CALCULATIONS

The path to accurate relativistic atomic structure calculations is lined with several points of interest or danger. Much of the non-relativistic many-body perturbation theory formalism, including the 'coupled-cluster approach'', can be taken over, but certain problems, such as the choice of electron-electron interaction, the need for projection operators and an apparent gauge dependence, must be dealt with. Results for the ground state energy of beryllium and also 2P-1,3-2S-1(0) transition energies in beryllium-like Fe and Mo are presented as examples, where the accuracy is such that the calculation of QED effects in many-electron systems might be tested. In addition, the application to the study of additional perturbations in alkali-like system is discussed.