Part (a) of IUPAC's definition says that the chemical bond simply is a force that binds atoms. If energy must be spent to stretch apart two atoms, then they are bonded. The definition does not expand on which kind of force, and therefore any force will be valid, from strong covalent to feeble London. Interestingly, the force must be exactly zero in a "stable independent molecular entity", or the bonds would continuously shrink until reaching a singularity (see Fig. ). Mathematically, with V being the potential energy and r the distance between two atoms: