[Wien] I2/a to b2/b space group

[Rocquefelte]

Dir Ali Hamidani,

Depending of the setting of the space group the cell parameters could 
change. In the present case, you have a monoclinic symetry and your are 
changing a "I" centering into a "B" centering.
Just draw the cell on a paper and you will see that such a change induce 
modification of cell parameters (you should also notice that the angle 
value has also changed).
Also if the cell you are using is different, you are describing the same 
solid. A change of setting do not lead to a change of the solid your are 
describing.

Concerning your question related to the band gap value, DFT is a 
fundamental state theory. To describe excited properties you should go 
behond DFT. However, in a qualitative way you can use the DFT states to 
describe XPS, optics ... But you should never forget that by using LDA 
or GGA we do not expect to have the experimental band gap. Usually, the 
band gap is underestimated (LDA, GGA). In some specific situations, you 
can use LDA+U or hybrid functionals to correct some deficiencies present 
in LDA. More details are available in the user guide.

With my best regards

Xavier



ali hamidani a écrit :
>
> Dir all
>
> I have used an external program to convert the I2/a to b2/b space 
> group for monoclinic compound (I think I2/a space group no yet 
> implemented in wien2k).
>
> My question is: are the both structures gave the same properties; 
> because I have found the lattice parameter is changed and I think the 
> electronic properties affected by this parameter. In another hand I 
> have found the band gap is 0 eV, in contrast with the experimental 0.7 
> eV. Is this due to the change of structure or from the approximation 
> used (LDA)
>
> Initial unit cell: (I2/a)
>
> 6.2070 5.8570 5.8740 90.000 111.800 90.000
>
> Final unit cell: (b2/b)
>
> 6.7787 6.2070 5.8570 90.000 90.000 126.430
>
>   
>
> Ali Hamidani
>
> Physics Laboratory at Guelma
>
> University 08 Mai 1945
>
> P.O. 401Guelma 24000
>
> Algeria
> ------------------------------------------------------------------------
>
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