Table VI. (Continued.) ∗ The results of Ref. [51] with the two Breit and one Coulomb photon-exchange contributions subtracted. 18 uncertainty we studied the convergence of the calculation depending on the configuration basis set and compared our results with very accurate data obtained for lithium with the variational solution of the three-body Schr¨ odinger problem that includes the relativistic cor- rections obtained within the αZ expansion [43, 54, 55]. The estimation of the residual three- and more photon-exchange QED effects is more difficult. As was found in Refs. [8, 13] the QED part of the two-photon exchange correction is anomalously small for the 2 s and 2 p$_{1}$$_{/}$$_{2}$ states. Moreover, the third order of the electron-electron interaction changes its sign when Z increases. Thus, the value based on the ratio of the two-photon exchange QED correction to corresponding non-QED contribution might underestimate the three-photon QED effects. For this reason, to estimate the uncertainty due to the QED effects, we take the ratio of the QED and non-QED two-photon contributions for the 2 p$_{3}$$_{/}$$_{2}$ − 2 s transition, where the QED effect is adequate, and multiply it by the maximal value of the third-order contribution among the 2 s , 2 p$_{1}$$_{/}$$_{2}$ and 2 p$_{3}$$_{/}$$_{2}$ states. Comparing the results for the third and higher orders ( E ⩾ $_{3}$) with the third order ( E$_{3}$ ), we conclude that corrections of the fourth and higher orders ( E ⩾ 3 − E$_{3}$ ) are rather important, especially for low- and middle- Z ions. We observe a reasonable agreement with Zherebtsov et al. [7] and Yerokhin et al. [13]. A small discrepancy with the results of Yerokhin et al. is caused by a different way of taking into account the Breit interaction. Yerokhin et al. treated the Breit interaction to first order Z Interaction Contribution 2 p$_{1}$$_{/}$$_{2}$ − 2 s 2 p$_{3}$$_{/}$$_{2}$ − 2 s Ref. 92 C+B E$_{3}$ 0.137 -0.041 92 C+B E$_{3}$ 0.160 [53] 92 C+B E$_{3}$ 0.131 [7] 92 C+B E$_{3}$ 0.167(23) [51] 92 C+B E$_{3}$ 0.147 ∗ [51] 92 C+B E$_{⩾}$$_{3}$ 0.137(7)(50) -0.039(7)(50)