Dear Rajdeep,
Raffaele Resta pointed out this earlier paper: http://journals.aps.org/prb/pdf/10.1103/PhysRevB.41.12358 . As you can see the problem is quite subtle, and for the time being it looks like you need to deal with a non-polar semiconductor. Another suggestion was to look at some of the recent work on flexoelectricity by Stengel, and by Vanderbilt - so in general you have a fairly complex (and thus exciting) problem at hand. nicola On 01/06/2014 15:59, Nicola Marzari wrote: > > > Dear Rajdeep, > > > good question, and I'd be intrigued to know the answer. > > First thing that comes to mind would be to do a reverse process > to what is used to calculate the work function. > > 1) in the bulk calculation, calculate both the VBM and the CBM, > and refer it to the planar average of the Hartree potential (a lot > of details here: http://infoscience.epfl.ch/record/32432, see fulltext > link, or in our 2009 PRB). This way, you know where the bands sit with > respect to another well defined (but still relative) bulk quantity. > > 2) now you need to determine where the average of the Hartree sits with > respect to the vacuum level - you look at a thick slab (appropriately > strained, to match uniaxial or biaxial strain above) and see where the > Hartree sits with respect to the vacuum level. This would still have > the problem that the different between Hartree and vacuum would be > affected by the surface (termination, resconstruction). But I guess > that is the case also in ARPES data - so I wonder if you really need > to go through all this process (I still think so, because as you > strain the crystal, the change you see in the VBM or CBM could be > in part physical, in part computational, due to the lack of an > absolute reference). > > If anyone has more experience in this, they'd be very welcome! > > nicola > > > On 01/06/2014 07:21, Rajdeep Banerjee wrote: >> Dear all, >> I want to look at the movement of valence band maxima (VBM / >> HOMO) and conduction band minima (CBM/LUMO) of a semiconductor for >> different values of applied tensile strain. The problem is how to fix a >> reference point to look at such a movement? >> 1. I can put Fermi energy as reference ... but for a semiconductor it >> doesn't mean anything as long as its in the band gap >> 2. I can put VBM as Fermi level and take that as reference ... but then >> I won't see VBM movement >> 3. I can use mid-point between HOMO and LUMO ... but then both the band >> edges will shift by same amount >> 4. I can fix an arbitrary point in the band gap as reference and check >> the movement without any shift ... but I'm a bit sceptical about this >> chice though. >> >> Please advise. Thanks in advance. >> >> warmest regards, >> Rajdeep Banerjee >> JNCASR >> Bangalore, India >> >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://pwscf.org/mailman/listinfo/pw_forum >> > -- ---------------------------------------------------------------------- Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
