Classical and Quantum Approaches

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, throughout the first part of this course, we shall use classical mechanics, as much as possible, to describe the translational motion of atoms and molecules, and will reserve quantum mechanics for dealing with non-translational motions. However, towards the end of this course, in Chapter 8, we shall switch to a purely quantum-mechanical approach.