That is, a stable geometry involves a minimum in V, the potential energy surface, and a zero force. In fact, a molecule is defined as a multiatomic entity in a minimum of the potential energy surface. Therefore, the definition of chemical bonds in a molecule in its ground state in terms of forces should be paradoxically expressed when the molecule is outside equilibrium, with stretched bonds. Note that the potential energy term here is not the Hamiltonian potential energy (obtained from the ɵ V operator in the ) and forces acting in a bond (dashed lines, right axis in kJ mol -1 Å -1 , see eq. 1 ). In a stationary geometry the forces are equal to zero by definition, since the potential energy of the molecule must be at its minimum. Note that by IUPAC's definition the hydrogens of ethene would be bonded, as there is a force hindering their separation.