https://doi.org/10.1351/goldbook.MT07073
The empirical function relating the potential energy of a molecule to the inter-atomic distance r accounting for the anharmonicity of bond stretching: \[E\left(r\right) = D_{\text{e}}\ (1- \text{e}^{-a\ (r- r_{\text{e}})})^{2}\] where \(D_{\text{e}}\) is the @B00699@, \(r_{\text{e}}\) is the equilibrium @BT07003@, and \(a\) is a parameter characteristic of a given molecule.
See:
harmonic approximation