[Pw_forum] hexagonal symmetry

[José Gadelha da Silva Filho]

Dear QE users,

I am working with a hexagonal symmetry crystal and I relaxed all internal
coordinates and cell parameters with vc-relax-bfgs algorithm.

I have some concern about my results. So, I would like some advice.

My initial cell parameter are:


45.48740530000000000 0.00000000000000000 0.00000000000000000
-22.74370265000000000 39.39324854203890000 0.00000000000000000
0.00000000000000000 0.00000000000000000 10.31035723491920000

with vectores  'a'  and 'b', as:

a = b = 45.48740530000000000
c = 10.31035723491920000

After optimization I got this value:

44.53851082397360000 0.00000000000000000 0.00000000000000000
-22.26925541198680000 38.57148182127630000 0.00000000000000000
0.00000000000000000 0.00000000000000000 10.12292974744850000

In my calculation all operations symmetry  are conserved. However if use
the cell parameters as:

a = 44,53851082397360000
b = 44,53851082482830000
c = 10,12292974744850000

The 'a'  and 'b' parameter aren't exactly the same.

I did a test, where I use

ibrav = 4
celldm(1)=  44.53851082397360000,
celldm(3) = 0.227284872 ,

The component forces of atoms become high than  (>10-3).

To keep the component forces of atoms less than >10-3,  I have to use ibrav
= 0 and specify the matriz of cell parameters.

With this process, Can I still consider my cell as hexagonal symmetry?

Cheers

-- 
José Gadelha da Silva Filho

Aluno de Doutorado do Curso de Pós-Graduação em Física
da Universidade Federal do Ceará.
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