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 $Z=5\text{–}30$. For the mixing $2{}^{1}P_1$ and $2{}^{3}P_1$ 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 $Z>10$.

• Received 28 June 2022
• Accepted 16 August 2022

DOI:https://doi.org/10.1103/PhysRevA.106.022815