M2L7f.txt

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The kinetic energy contribution is a simple expansion.
You can get it from the Pauli approximation.
You take the Dirac equation, deal
with all the operators in the Dirac equation
and do an expansion.
But you can also just lean back and say OK,
what is the kinetic energy?
And use the relativistic form the kinetic energy,
which is rest energy and pc squared, the square root of it.
And if you say the kinetic energy is the energy minus mc
squared, and then you take the full relativistic expression
for the total energy minus 1.
And now you do a Taylor expansion of the square root.
You find the non-relativistic kinetic energy
and the first correction term with all
the correct pre-factors
is this one.

So this is a relativistic energy correction
in the Pauli equation.
That's all we have to say.