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Hartree units
Remind that the Bohr radius is given by
{$$a_B = \frac {4\pi\varepsilon_0 \hbar^2}{e^2 m}$$}
We can therefore define the Hartree s twice the first ionization energy of hydrogen and rescale energies to this quantity
{$$H = \frac {\hbar^2}{m a_B^2} = \frac {2e^2}{4\pi\varepsilon_0 a_B}$$}
Let's introduce Hartree units: lengths are rescaled to the Bohr radius, {$a_B$}, hence space derivatives to its inverse and energyies to the Hartree
{$$ \begin{align*}
\mathbf r & \rightarrow a_B \mathbf r\\
\boldsymbol \nabla & \rightarrow \frac{\boldsymbol \nabla}{a_B}\\
E & \rightarrow \frac {\hbar^2}{m a_B^2} E
\end{align*}$$}
We can now rewrite the simplest many (two) body Hamiltonian, helium, as
{$${\cal H} = -\sum_{i=1}^2 \left(\frac {\nabla^2} 2 + \frac Z {r_i}\right) + \frac 1 {r_{12}}$$}
with {$Z=2$} and {$r_{12}=|\mathbf r_2-\mathbf r_1|$}.
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