Relativistic kinetic-energy correction
= Relativistic kinetic-energy correction
{title2=$\delta H=-p^4/(8m^3c^2)$}
Expanding the <relativistic kinetic energy> $\sqrt{m^2c^4+c^2p^2}-mc^2$ for $|p|\ll mc$ gives $p^2/(2m)-p^4/(8m^3c^2)+\cdots$. The first correction is negative and contributes to atomic <fine structure>. Its perturbative use requires small momentum compared with mc; the truncated negative quartic term is not a globally stable exact Hamiltonian.