[Pw_forum] interpretation of the file containing the real space force constants

[Eric Landry]

Dear users,

I'm trying to understand the format of the file containing the real space
force constants output from the q2r.x program. Here is a selection of the
output file for calculation of Si using a (444) grid:

4   4   4
1   1   1   1
1   1   1   2.71632979375E-01
2   1   1  -3.79717062500E-03
3   1   1   3.75093750000E-05
4   1   1  -3.79717062500E-03
1   2   1   9.08752437500E-03
2   2   1  -3.62702500000E-04
3   2   1   3.30887500000E-05
4   2   1  -3.79717062500E-03
1   3   1   4.08761312500E-03
2   3   1  -3.62702500000E-04
3   3   1   3.75093750000E-05
4   3   1  -3.62702500000E-04 ... 

I believe I understand the lines like "1 1 1 1" occurring above, but what
are the three numbers before the value of each force constant (how do these
numbers correspond to the atomic displacements?) Also, what are the units
given for the force constant?

Thanks in advance,

Eric Landry, 
Ph.D. candidate 
Department of Mechanical Engineering, Carnegie Mellon University