ABSTRACT
This chapter is based on non-relativistic quantum mechanics. Although it is a good approximation for light atoms, relativistic effects must be included even in these atoms or ions in order for the predictions of the theory to be in detailed agreement with experiment. When moving on to heavy ions, or highly ionized systems, the importance of relativistic effects increases quickly. For a general computational method, whose aim is to be valid for a large vanety of systems, it is therefore essential to take relativistic effects into account. Glass and Hibbert in 1978 observed that in many-electron system the matrix elements of the different fine structure operators have different dependence on the quantum numbers involved. The Blume-Watson approach is implemented in the BREIT program. It is important to remember that it is inconsistent to include the spin-orbit effect, without including the spin-other-orbit, in any system with a core.
