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 De is the bond-dissociation energy, r e is the equilibrium bond length, and a is a parameter characteristic of a given molecule.