Abstract
The approximation to the electron self-energy has become a standard method for ab initio calculation of excited-state properties of condensed-matter systems. In many calculations, the self-energy operator, , is taken to be diagonal in the density functional theory (DFT) Kohn-Sham basis within the scheme. However, there are known situations in which this diagonal approximation starting from DFT is inadequate. We present two schemes to resolve such problems. The first, which we called , involves construction of an improved mean field using the static limit of , known as COHSEX (Coulomb hole and screened exchange), which is significantly simpler to treat than . In this scheme, frequency-dependent self energy , is constructed and taken to be diagonal in the COHSEX orbitals after the system is solved self-consistently within this formalism. The second method is called off diagonal-COHSEX (). In this method, one does not self-consistently change the mean-field starting point but diagonalizes the COHSEX Hamiltonian within the Kohn-Sham basis to obtain quasiparticle wave functions and uses the resulting orbitals to construct the in the diagonal form. We apply both methods to a molecular system, silane, and to two bulk systems, Si and Ge under pressure. For silane, both methods give good quasiparticle wave functions and energies. Both methods give good band gaps for bulk silicon and maintain good agreement with experiment. Further, the method solves the qualitatively incorrect DFT mean-field starting point (having a band overlap) in bulk Ge under pressure.
- Received 11 September 2013
- Revised 25 July 2014
DOI:https://doi.org/10.1103/PhysRevB.90.115148
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