= Darwin term
{c}
{title2=$H_D=\hbar^2\nabla^2V/(8m^2c^2)$}
The spin-one-half low-energy expansion of the <Dirac equation> includes this local correction in a static potential energy V. For $V=-Ze^2/(4\pi\epsilon_0r)$, $\nabla^2V=Ze^2\delta^{(3)}(\mathbf r)/\epsilon_0$, 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>. <Topics in Quantum Mechanics by David Tong>[§3.1.4, printedpp.76–78]{ref}
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