Quasiparticle band structure and density-functional theory: Single-particle excitations and band gaps in lattice models

We compare the quasiparticle band structure for a model insulator obtained from the fluctuation exchange approximation (FEA) with the eigenvalues of t he corresponding density-functional theory (DFT) and local-density approxim ation (LDA). The discontinuity in the exchange-correlation potential for th is model is small; and the FEA and DFT band structures are in good agreemen t. In contrast to conventional wisdom, the LDA for this model overestimates the size of the band gap. We argue that this is a consequence of an FEA se lf-energy that is strongly frequency dependent, but essentially local. [S01 63-1829(99)00123-X].