The energy levels of helium atom

In our work the nonrelativistic ionization energies of helium atom are calculated for S-, P- and D-states. The calculations are based on the variational method of 'exponential' expansion. Convergence of the numerical values of variational energies is studied by increasing the number of the basis functions N. That allows to claim that the obtained energy values have 20 significant digits, both for S states and for states of nonzero angular moment