Wannier Koopman method calculations of the band gaps of alkali halides

Correcting the band structure within the density functional theory (DFT) formalism is a long term goal for its development. Recently, we have proposed a Wannier Koopman method (WKM) to correct the DFT bandgap using the Kohn-Sham equation. Previous tests show that WKM works well for common semiconductors. Here, we test its accuracy in terms of predicting the bandgap of extreme ionic crystals: alkali halides. We found that the WKM can accurately reproduce the alkali halide bandgaps with accuracy in par with the GW method. On the other hand, the hybrid functional with common parameters, which work well for common semiconductors, significantly underestimate the alkali halides.