Breaking covalent bonds using mechanical force, which bond breaks?

A constrained geometry optimisation scheme in combination with density functional theory is applied to probe the effect of an external force on a molecule. When the external force overcomes a critical value, one bond will break. In a polyatomic molecule, it is of special interest to see which bond breaks and why. In the present work, a series of organic molecules have been studied and it is found that it is not necessarily the thermodynamically weakest bond that breaks. It is found that the dissociation energy, the harmonic force constant and the angle between the given bond and the external force are all important factors in this process.