Two electrons “a” and “b” move independently of each other and the total energy of the system is simply sum of the energies of non-interacting particle.



The ground state energy of Helium (when both the electrons are in 1s level)


The experimental value of the ionization energy i.e. the energy required to remove one electron from Helium atom = 24.63 eV
When one electron is removed then the energy of ionized Helium ion (He+)
                                               = -Z2´13.6 eV =-54.4 eV
So the ground state energy of Helium = -54.5 eV - 24.63 eV = -79.03 eV
Thus, Atomic energies calculated in this manner are the poor approximation to the actual eigen values.
However, the calculated energies and wavefunctions can be improved by introducing the interelectronic repulsion terms  as a perturbation.