The extent to which the oscillation of an oscillator differs from simple harmonic motion. In molecular vibrations the anharmonicity is very small near the equilibrium position, becomes large as the vibration moves away from the equilibrium position, and is very large as dissociation is approached. Anharmonicity is taken into account in molecular vibrations by adding an anharmonicity term to the potential energy function of the molecule. For a harmonic oscillator the potential energy function U is given by U=f(r – re)2 where r is the interatomic distance, re is the equilibrium interatomic distance, and f is a constant. Anharmonicity is taken into account by adding a cubic term g(r – re)3 to the quadratic term, where g is much smaller than f. Higher terms in (r – re) can be added to improve the description of anharmonicity.