Turning now to quantum mechanics an atom or a molecule can be thought of as a collection of point charges (electrons and nuclei), so that the second definition of the dipole applies. The interaction of atom or molecule with a uniform external field is described by the operator V i n t = − − F ⋅ ⋅ μ μ . This operator is used as a perturbation in first- and second-order perturbation theory to account for the first- and second-order Stark effect.