Journal of Physics B: Atomic and Molecular Physics
The leading two terms in the 1/Z expansion of the two-electron Bethe logarithm are calculated by the application of a new finite basis set method. The results can be expressed in the form ln epsilon (1s 2 1S)=ln(19.77(Z-0.0063) 2). The high-Z behaviour appears to differ from that of a previous variational calculation by Aashamar and Austvik (1976).
Goldman, S. P. and Drake, Gordon W. F.. (1983). 1/Z expansion calculation of the Bethe logarithm for the ground state Lamb shift of two-electron ions. Journal of Physics B: Atomic and Molecular Physics, 16 (7), L183-L187.