The document discusses the harmonic oscillator model of molecular vibration. It explains that atoms in molecules periodically displace from their equilibrium positions, changing bond lengths and angles in a simple harmonic motion. This periodic motion results in changes to the vibrational and rotational energy levels. The harmonic oscillator model treats vibrating diatomic molecules as simple harmonic oscillators that obey Hooke's law, where a restoring force is proportional to the displacement from the equilibrium position. The frequency of this periodic motion is known as the vibrational frequency. Quantum mechanical equations like the Schrödinger equation can calculate the allowed vibrational energies.