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?


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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

2017-08-07 8:38 GMT+02:00 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
      > 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
      > 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

      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
      can be accepted that doing a calculation can sustitute
      performing an
      experiment. When designing a building or a bridge. Nobody would
      that a good resistence analysis is missed, but that means that
      resistence is well known in advance.

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

      Also the examination of families of compounds in looking for the
      promising candidates can be of great help, even in early stages
      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
       /`. \/ .'\
      /   '`'`   \
      |  \'`'`/  |
      |  |'`'`|  |
      ! Dr.Víctor Luaña, in silico chemist & prof. !
      ! Departamento de Química Física y Analítica !
      ! Universidad de Oviedo, 33006-Oviedo, Spain !
      ! e-mail:  <>  !
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