[Pw_forum] The output of d3x

[Hua Bao]

Dear All,

I am working on the anharmonic force constant calculation following example
14.
Since there is not much description of the output format (let me know if I
am wrong), I have to guess what the parameters are.

Based on Debernardi's thesis and the discussions,
I guess the code calculates  C(0,q,-q), but this quantity is also a
function of p, p', p'', which are the components of atomic displacements.

I have read the code a little bit. The d3 matrix is stored in the quantity
phi, which is a function of atom index and direction index( i guess?).

However, I could not understand the output format as follows

   modo:    1

  1  1
     -0.138777878078E-15      0.000000000000E+00     -0.832667268469E-16
      0.000000000000E+00     -0.277555756156E-16      0.000000000000E+00
     -0.416333634234E-16      0.000000000000E+00      0.832667268469E-16
      0.000000000000E+00      0.383064069101E+00      0.000000000000E+00
     -0.111022302463E-15      0.000000000000E+00      0.383064069101E+00
      0.000000000000E+00     -0.277555756156E-16      0.000000000000E+0

I cannot understand "modo" and (1,1).
I suspect (1 1) means atom index, but there are p, p', p''. Which two (1,1)
stands for?

Also, in the README file. It shows something like C_{x,y,z} (0,0,0|1,1,1).
I can understand (0,0,0) are the three q-points, but what does this 1,1,1
stands for? It does not look like phonon branch index or atom index.

I have been struggling on this for quite some time. I really appreciate if
anyone can help me to clarify this issue.

Thanks,
Hua
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