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 1996 Comput. Phys. Commun. 94 249) multi-configuration Dirac-Fock (MCDF) program 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 helium 1 S ground state isoelectronic sequence. We also employ a multi-configuration Hartree-Fock (MCHF) method to calculate the ground state SMS for comparison with MCDF results. The SMS matrix elements for two-electron systems obtained from the relativistic program are shown to exhibit a trend: the larger the atomic number Z , the larger the relativistic contributions to the SMS matrix elements for the ions. The SMS matrix elements approximately vary as Z 3 along the isoelectronic sequence from Z = 2 to 92. In addition, it is shown that the relativistic effects increase approximately as Z 3 / A 2 in the SMS values for all the ions considered confirming some previous observations (Parpia F A, Tong M and Froese Fischer C 1992 Phys. Rev. A 46 3717-24). Excellent agreement is found between the present ab initio calculations and the available semi-relativistic calculations for the small values of Z along the helium-like ions. Furthermore, a large set of configuration state functions used in the calculations has revealed larger disagreements for high- Z ions between both relativistic (MCDF-optimized-level) and nonrelativistic (MCHF) calculations suggesting that the SMS for helium-like ions with Z >40 relativistic and correlation effects are increasingly important.