[Pw_forum] how to judge movement of HOMO and LUMO

[Nicola Marzari]

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