4. Bond force constants

A diatomic molecule $A\text{—}B$ , whose bond length is r _e , is not a rigid whole, but is animated by an oscillatory motion around its equilibrium geometry. The potential energy curve of the molecule $A\text{—}B$ has a minimum for an interatomic distance r equal to r _e , which corresponds to a stable equilibrium.

In the vicinity of r _e , i.e. in the approximation of small displacements, the potential energy can be represented by a parabola of equation :