[Pw_forum] Band gap hybrid functionals
myoussef at mit.edu
Wed Nov 11 18:03:16 CET 2015
Dear Phil and all,
Note that for a functional with exact exchange, the difference between ionization potential and electron affinity (I-A) is not the same as the Kohn-Sham single particle states gap. The consensus, as far as I know, is that I-A is more physical and relevant to comparison with experimental band gaps. Calculating I-A requires 3 scf calculations (neutral cell, +electorn, -electron). The cases of +/- e require high smearing similar to that of metals (or you could use the tetrahedron method but I doubt it is implemented for hybrids). Also +/- calculations require large cells and correction for a charged cell (Makov-Payne for example). See section III-A in this paper for elaboration on computational details:
PHYSICAL REVIEW B 78, 235104 ?2008?
For PBE, LDA, ... I-A is the same as Kohn-Sham gap.
For elaboration on the theoretical aspects and why I-A is not the same as K-S band gap for the exact exchange, consult the work of A. J. Cohen and collaborators .
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