We apply the quasiparticle self-consistent GW method (QSGW) to slab models of ionic materials, LiF, KF, NaCl, MgO, and CaO, under electric field. Then we obtain the optical dielectric constants E(Slab) from the differences of the slopes of the electrostatic potential in the bulk and vacuum regions. Calculated E(Slab) show very good agreements with experiments. For example, we have E(Slab)=2.91 for MgO, in agreement with the experimental value E(Experiment)=2.96. This is in contrast to E(RPA)=2.37, which is calculated in the random-phase approximation for the bulk MgO in QSGW. After we explain the difference between the quasiparticle-based perturbation theory and the Greens function based perturbation theory, we interpret the large difference E(Slab)-E(RPA)=2.91-2.37 as the contribution from the vertex correction of the proper polarization which determines the screened Coulomb interaction W. Our result encourages the theoretical development of self-consistent G0W approximation along the line of QSGW self-consistency, as was performed by Shishkin, Marsman and Kresse [Phys. Rev. Lett. 99, 246403(2007)].
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