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.
si.phG.in
Description: Binary data
si.q2r.in
Description: Binary data
si.scf.in
Description: Binary data
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