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

[Karel Vyborny]

In my opinion, you can't say that. Some authors would do this but I don't 
think this is a good scientific practice. You can present results of all 
four calculations of course, but then it is just to say that regarding the 
size of the gap ("positive or negative") the results are inconclusive. It 
may be helpful to compare the four band structures and see how similar 
they are. But simply, when it comes to the precise position of individual 
bands (are they shifted couple of 0.1 eVs up or down) you don't have the 
necessary accuracy. Also note that the value of U in one of your 
calculatations will probably have a strong influence - do you have any 
idea about reasonable range of this parameter?

KV


--- x ---
dr. Karel Vyborny
Fyzikalni ustav AV CR, v.v.i.
Cukrovarnicka 10
Praha 6, CZ-16253
tel: +420220318459


On Mon, 7 Aug 2017, Wien2k User wrote:

> thank you for your answers
> 
> But for my problem is what I have to say that the material is semimetalic
> since this result is obtained with GW, GGA+U and mBJ?.
> Or I have to present all the results even those obtained by HF and I would
> say that the calculations gave different results pending confirmation of the
> experiment?
> 
> 
> 2017-08-07 8:38 GMT+02:00 Víctor Luaña Cabal
> <victor at fluor.quimica.uniovi.es>:
>       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
>       --
>           .  .    "Half of the US people use twitter to form its
>       opinion and half
>          / `' \   also elect the US president. I only hope they are
>       not the same
>         /(o)(o)\  half". --From a sentence by Gore Vidal
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>       ==(((==)))===================================+===========================
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>       ! Departamento de Química Física y Analítica !
>       ! Universidad de Oviedo, 33006-Oviedo, Spain !
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