[Pw_forum] Using q2r.x - incorrect IFC symmetries

[Navaneetha Krishnan]

Forgot to attach my IFC file. Please find si.555.fc attached with this
email.

Thanks,
-Navaneeth

On Mon, Feb 9, 2015 at 3:45 PM, Navaneetha Krishnan <navaneeth at caltech.edu>
wrote:

> Hello Quantum Espresso users,
>
> I am trying to use QE to get the real space harmonic force constants using
> pw.x, ph.x and q2r.x in the same order for Si. I have attached all my input
> files. I am using a 5X5X5 mesh.
>
> I used ibrav=0 and my lattice vectors are [0 0.5 0.5], [0.5 0 0.5], [0.5
> 0.5 0] in alat units. So if I understand the convention given in the faq
> phonon section of the documentation, the first nearest neighbors should
> have the indices [1 1 1], [5 1 1], [1 5 1] and [1 1 5].
>
> From my understanding, the first nearest neighbours for Si have the
> diagonal term in the force constant matrix +/- alpha and off-diagonal term
> in the force constant matrix +/- beta for some alpha and beta.
>
> Now, the [1 1 1] force constant seems to have the right symmetries :
> atom pair : 1 2
> 148    1   1   1  -6.40122756000E-02 11
> 652    1   1   1  -4.29620484800E-02 12
> 1156    1   1   1  -4.29620484800E-02 13
> 1660    1   1   1  -4.29620484800E-02 21
> 2164    1   1   1  -6.40122756000E-02 22
> 2668    1   1   1  -4.29620484800E-02 23
> 3172    1   1   1  -4.29620484800E-02 31
> 3676    1   1   1  -4.29620484800E-02 32
> 4180    1   1   1  -6.40122756000E-02 33
>
> However, the [5 1 1] (and [1 5 1], [1 1 5]) force constants do not have
> the right symmetries
> atom pair : 1 2
> 152    5   1   1  -3.87971851534E-03 11
> 656    5   1   1   1.05471196450E-03 12
> 1160    5   1   1   1.05471196450E-03 13
> 1664    5   1   1   1.05471196450E-03 21
> 2168    5   1   1  -3.60176689065E-04 22
> 2672    5   1   1  -3.96403112877E-04 23
> 3176    5   1   1   1.05471196450E-03 31
> 3680    5   1   1  -3.96403112877E-04 32
> 4184    5   1   1  -3.60176689065E-04 33
>
> *The first column is the line number in my file, then followed by the
> three indices for the lattice vectors, then the force constant and then
> finally the component (11=xx, 12=xy and so on)*
>
> Does anyone have an idea why this happens?
>
> P.S. : In the si.scf.in file, I have CELL_PARAMETERS {cubic}. I have also
> tried with CELL_PARAMETERS {alat} and there is no difference.
>
> Thanks,
> Navaneeth
> --
> Navaneetha Krishnan Ravichandran,
> Graduate Student,
> Mechanical Engineering,
> Caltech.
>



-- 
Navaneetha Krishnan Ravichandran,
Graduate Student,
Mechanical Engineering,
Caltech.
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