 Darwin term

The spin-one-half low-energy expansion of the Dirac equation includes this local correction in a static potential energy V. For $V=-Z{e}^2/(4\pi {ϵ}_{0}r)$, ${\nabla }^{2}V=Z{e}^2{\delta }^{(3)}(\mathbf{r})/{ϵ}_0$, so the correction is positive and probes the wavefunction at the origin. In point-Coulomb hydrogen atom states it contributes only to S waves at leading nonrelativistic order. It is part of fine structure, distinct from the radiative Lamb shift. ^[ref]