Abstract
A calculation of two-electron QED effects to all orders in the nuclear binding strength parameter Z alpha is presented for the ground and n = 2 excited states of heliumlike ions. After subtracting the first terms of the Z alpha expansion from the all-order results, we identify the higher-order QED effects of order m alpha 7 and higher.
Combining the higher-order remainder with the results complete through order m alpha 6 from Yerokhin and Pachucki [Phys. Rev.
A 81, 022507 (2010)], we obtain the most accurate theoretical predictions for the ground and nonmixing n = 2 states of heliumlike ions with Z = 5-30. For the mixing 2 1P1 and 2 3P1 states, we extend the previous calculation by evaluating the higher-order mixing correction and show that it defines the uncertainty of theoretical calculations in the LS coupling for Z > 10.