[Pw_forum] Real space force constants file

[Paolo Giannozzi]

I just added point 7.7 of the FAQs, since this question is asked
more often than not: http://www.quantum-espresso.org/faq/phonons/#7.7

P.

On Tue, 2014-05-27 at 09:20 +0530, Sridhar Sadasivam wrote:
> 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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