[Wien] gap

[Parker, David S.]

Ben, the reason there are three different band gaps listed is that there are three different k-point meshes used for the scf, band structure and density of states calculations (although one can for this use the same mesh as for scf).  WIEN can only tell you the difference  between the highest occupied eigenvalue and the lowest unoccupied eigenvalue, for the chosen k-point mesh.  In the limit of an infinitely fine k-mesh this will be the band gap, but of course any practical k-point mesh will have a certain finite mesh grid.

What I would recommend doing is to use xcrysden on a relatively fine k-point mesh (maybe 20x20x20 at minimum) to plot the Fermi surface to determine the location in k-space of the valence band maximum and conduction band minimum.  Often but not always this will be at a symmetry point; you can then select a path in k-space that includes this point (ie. case.klist_band).  Xcrysden will actually tell you what the band range is (use a fine k-point mesh) and this will give you a good idea of the gap.

Note also that if the VBM and/or CBM do not fall on a symmetry point, the gap reported in case.scf (or case.scf2) will virtually always be an overestimate of the real gap, and the problem becomes more severe for very dispersive bands. I do a lot of calculations for very small gap semiconductors, like 0.1 eV or less, and frequently the gap given in case.scf can be 0.1 or even 0.2 eV larger than the actual gap.  To sum up: when in doubt, plot the Fermi surface in xcrysden and locate the VBM and CBM, then choose a case.klist_band mesh that includes these points.  Good luck - David Parker

From: wien-bounces at zeus.theochem.tuwien.ac.at [mailto:wien-bounces at zeus.theochem.tuwien.ac.at] On Behalf Of ben amara imen
Sent: Wednesday, October 17, 2012 11:26 AM
To: wien at zeus.theochem.tuwien.ac.at
Subject: [Wien] gap

Hello !

 I want to determine the gap's value for my compound. So, I have calculated DOS and structure bands SB. In my case, DOS gives a value , SB gives me  another different value and I have found a another value in case.SCf2 . So I have a three different value of gap !!!!

I want to know the singnification of each value (given by DOS; SB and case.SC2 ) and what's the right value??? I mean what the right value  that I have to choose

Can someone helps me please!
 And thanks in advance
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