MCDF calculations of the specific mass shift in helium-like ions

An extension of the GRASP92 (Parpia F A, Froese Fischer C and Grant I P 199 6 Comput. Phys. Commun. 94 249) multi-configuration Dirac-Fock (MCDF) progr am described previously (Perger W F and Idrees M 1995 Phys. Commun. 85 389- 97) is used for the calculation of the specific mass shift (SMS) of the hel ium S-1 ground state isoelectronic sequence. We also employ a multiconfigur ation Hartree-Fock (MCHF) method to calculate the ground state SMS for comp arison with MCDF results. The SMS matrix elements for two-electron systems obtained from the relativistic program are shown to exhibit a trend: the la rger the atomic number Z, the larger the relativistic contributions to the SMS matrix elements for the ions. The SMS matrix elements approximately var y as Z(3) along the isoelectronic sequence from Z = 2 to 92. In addition, i t is shown Z(3)/A(2) in the SMS values for that the relativistic effects in crease approximately as;ii. all the ions considered confirming some previou s observations (Parpia F A, Tong M and Froese Fischer C 1992 Phys. Rec: A 4 6 3717-24), Excellent agreement is found between the present ab initio calculations and the available semi-relativistic calculations for the small values of Z alo ng the helium-like ions. Furthermore, a large set of configuration state fu nctions used in the calculations has revealed larger disagreements for high -Z ions between both relativistic (MCDF-optimized-level) and nonrelativisti c (MCHF) calculations suggesting that the SMS for helium-like ions with Z > 40 relativistic and correlation effects are increasingly important.