[Pw_forum] Real space force constants file

[Sridhar Sadasivam]

Dear QE users and developers,

I am trying to use the real space force constants file obtained from q2r.x
to do some other calculations outside of QE. For this purpose, I am trying
to understand the format of the force constants file. I am attaching a
sample force constants file obtained by performing ph.x and q2r.x on bulk
silicon with a 4 x 4 x 4 q-mesh.

In the 22nd line in the file, it says 1 1 1 1. I believe the first two 1's
denote the x-direction and the last two 1's denote atom numbers within the
two-atom unit cell of silicon. After this it loops over all 64 different
combinations of indices in a 4 x 4 x 4 mesh and lists the force constants
in the 4th column. What do these 64 different combinations mean? Do they
denote unit cells?

For instance, let's take the first line: 1 1 1 2.6997. 2.6997 is the force
constant between which atoms?

Thanks for your help!
Sridhar
Purdue University
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