It would be very useful to have the Hartree (Ha or Eh) and Rydberg (Ry) energy units implemented. The relation between these is trivial
1 Ha = 2 Ry
and the Hartree is
1 Ha = electron_mass⋅(elementary_charge^2/(4⋅pi⋅electric_constant⋅planck2pi))^2
~ 27.21 eV
