무고조파

thumb|Potential energy of a diatomic molecule as a function of [[atomic spacing. When the molecules are too close or too far away, they experience a restoring force back towards u0. (Imagine a marble rolling back and forth in the depression.) The blue curve is close in shape to the molecule's actual potential well, while the red parabola is a good approximation for small oscillations. The red approximation treats the molecule as a harmonic oscillator, because the restoring force, ''-V'(u), is linear with respect to the displacement u''. ]]