With a limit on the small number of problems that admit exact
solutions, we turn in this chapter to approximation methods that can be used
for bound state problems. The most commonly used method is the perturbation
theory for which one use a small parameter to form the basis of a systematic
expansion, or a variational method in which one hopes to set the best limit on
the energy of a system. We will consider both of these approximation methods
with applications to problems in atomic and molecular physics.
