Title: Helium-like and Lithium-like ions: Ground state energy,
Abstract: 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 $Z$.