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We have seen, in Sect. 9.4, that we can solve Schrödinger's equation *exactly* to find the stationary eigenstates
of a hydrogen atom. Unfortunately, it is not possible to find exact
solutions of Schrödinger's equation for atoms more complicated than hydrogen, or for molecules. In such systems, the best that we can do
is to find *approximate* solutions. Most of the methods which have been developed for finding such solutions employ the so-called *variational
principle* discussed below.

Richard Fitzpatrick
2010-07-20