[Pw_forum] how to judge movement of HOMO and LUMO

[Nicola Marzari]

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
>>
>>
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>

-- 

----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL