Morse

[classi] [classj] morse [eps] [rho] [r_e]

$$U_{\mathrm{Morse}} = \epsilon \left[1 - \exp\left\{ -\rho \left(r - r_e\right) \right\} \right]^2 - 1$$ (7)

where $\epsilon$ is the energy scale, $\rho$ is the range parameter, and $r_e$ is the equilibrium bond distance (or length scale).