[Wien] Gap calculated with GGA+U, mBJ and HF

[Víctor Luaña Cabal]

On Mon, Aug 07, 2017 at 01:32:04AM +0200, Wien2k User wrote:
> I calculated the gap of a material whose experimental value is unknown.
> 
> With GGA+U, mBJ potential and GW, I found that this material is semimetal
> (a slight band overlap) but with HF i found it to be semiconductor of very
> small gap.
> 
> How can I know what is the real nature of the gap of this material?
> 
> This calculation is a part of a future paper so I have to put all found
>  results (GGA + U, mBJ, GW and HF) in spite of what give deferent nature or
> I have to adopt that of the HF since it is a more accurate method?

This is a very good question related to the nature and philosophy of
theoretical calculations.

Assuming that the role of a theoretical calculation is predicting or
duplicating the experimental results is a dead way when the theory is yet
imprecise or incompletely developed.

In fact, as theory matures it becomes a part of engineering and it
can be accepted that doing a calculation can sustitute performing an
experiment. When designing a building or a bridge. Nobody would accept
that a good resistence analysis is missed, but that means that materials
resistence is well known in advance.

Until theory reaches that point of development, the role of a good
calculation is determining how dependent are the obtained properties
to the inner parameters of the calculation. So, in your example,
I would give more value to exploring the effects and meaning of the
different methods.

Also the examination of families of compounds in looking for the most
promising candidates can be of great help, even in early stages of
a research.

Hope this helps,
                Víctor Luaña
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