[Pw_forum] Structuring CELL_PARAMETER

[Lorenzo Paulatto]

On 29/09/17 09:28, Omamuyovwi Akemu wrote:
> Dear QE users,
> 
> I apologize if the question seem trivial. I have searched the pw-forum 
> archive for a solution but did not get any.
> 
> I want to set up the input of a BCC material using ibrav= 0
> When I set ibrav=3, the structure looks okay from xcrysden

What do you mean by "ok"? Does it look like a cube with many atoms in 
it, or like the actual unit cell of the BCC lattice ?

Try pressing F3 and F4 to switch between the conventional cubic cell and 
he actually BCC unit cell. Does this solve your problem?

I've attached two images of BCC Iron, for exactly the same input, one is 
the conventional cell, the other is the BCC cell. Note that the 
conventional cell has twice as many atoms, i.e. it is a supercell



> &SYSTEM
>    ibrav       = 3,
>    celldm(1)   = 6.475745143,
>    nat         = 1,
>    ntyp        = 1,
> 
> 
> But with ibav=0 and the CELL_PARAMETER card included, the structure 
> looks different (not BCC).
> &SYSTEM
>    ibrav       = 0,
>    celldm(1)   = 6.475745143,
>    nat         = 1,
>    ntyp        = 1,
> .....
> CELL_PARAMETERS
>    0.500000   0.500000   0.500000
>   -0.500000   0.500000   0.500000
>   -0.500000  -0.500000   0.500000
> 



> 
> However, I noticed that with ibrav=0 and
> CELL_PARAMETERS
> 1.0   0.0   0.0
> 0.0   1.0   0.0
> 0.0   0.0   1.0
> using the vectors of a simple cubic material and ntyp =2 at  atomic 
> postions 0,0,0 and 1/2,1/2,1/2, the structure is seen as BCC from xcrysden.
> 
> I desire to use ibrav=0 and the lattice vectors of a BCC crystal.
> I will sincerely appreciate any comment that can help me proceed with my 
> calculation.
> 
> Thank you.
> 
> Jolayemi Omamuyovwi RIta
> Research Student
> University of Benin
> Nigeria.
> 
> 
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-- 
Lorenzo Paulatto - Paris
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