Helium-like and Lithium-like ions: Ground state energy

It is shown that the non-relativistic ground state energy of helium-like and lithium-like ions with static nuclei can be interpolated in full physics range of nuclear charges Z with accuracy of not less than 6 decimal digits (d.d.) or 7-8 significant digits (s.d.) using a meromorphic function in appropriate variable with a few free parameters. It is demonstrated that finite nuclear mass effects do not change 4-5 s.d. for Z in [1,50] for 2-,3-electron systems and the leading relativistic and QED corrections leave unchanged 3-4 s.d. for Z in [1,12] in the ground state energy for 2-electron system, thus, the interpolation reproduces definitely those figures. A meaning of proposed interpolation is in a construction of unified, {\it two-point} Pade approximant (for both small and large Z expansions) with fitting some parameters at intermediate Z.