Morse potential

The empirical function relating the potential energy of a molecule to the inter-atomic distance r accounting for the anharmonicity (See: harmonic approximation) of bond stretching:

$E r = D e ( 1 − e − a ( r − r e ) ) 2$

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

Source:

PAC, 1999, 71, 1919 (Glossary of terms used in theoretical organic chemistry) on page 1953