An anharmonic oscillator is a model used in physics and chemistry to describe a system where the restoring force is not proportional to the displacement from the equilibrium position. This non-linearity means that the energy levels are not evenly spaced, which contrasts with a harmonic oscillator where they are. In the context of vibrational energy levels and modes, anharmonic oscillators provide a more accurate representation of real molecular vibrations, as molecules experience potential energy surfaces that deviate from the ideal harmonic approximation.
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