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We mentioned earlier that the
motions (by which we really meant the translational motions)
of atoms and molecules are described exactly
by quantum mechanics, and only approximately by classical mechanics.
It turns out that the non-translational motions
of molecules, such as their rotation
and vibration, are very poorly described by classical mechanics.
So, why bother using classical mechanics at all? Unfortunately, quantum
mechanics deals with the translational motions of atoms and
molecules (via wave mechanics) in a rather awkward manner. The
classical approach is far more straightforward, and, under most
circumstances, yields the same statistical results. Hence, in the
bulk of this course,
we shall use classical mechanics, as much
as possible, to describe translational motions, and reserve quantum
mechanics for dealing with non-translational motions. However, towards
the end of this course, we shall switch to a purely quantum mechanical
approach.

Richard Fitzpatrick
2006-02-02