The experimental internuclear potential energy curve is approximated using a Morse potential and it is defined as follows:

U(x) = D ( 1 - e^{-b(x - x0 )})²

The internuclear forces can be determined by calculating the negative gradient of U(x) with respect to the atomic coordinates of the second atom, whose position is controlled using the optical tweezer.

For small displacements away from the equilibrium bond length, the magnitude of the internuclear force can be approximately written as

F (x) = - K( x - x₀ )