[Pw_forum] Using q2r.x - incorrect IFC symmetries

[Navaneetha Krishnan]

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.
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