[Pw_forum] *.fc file in PW_SCF help.

Stefano de Gironcoli degironc at sissa.it
Sat Aug 1 09:54:56 CEST 2009

```there s one confusing bit that maybe needs to be clarified ...
The definition that have been quoted
> 4 4 4 - you used 4x4x4 q-points to generate FC
> 1 3 1 2 - x, z, 1st atom, 2nd atom
> 4 3 2 0.1234 - R=i*a+j*b+k*c, where i,j,k are integers from 1 to 4 (you used 4x4x4 mesh), a,b,c are lattice vectors. 0.1234 is F_{1,2}^{x,z}(R(4,3,2)).
>
> As QE uses atomic units, F should be in units of Ry/au^2, but not in eV/A^2.
is ALMOST correct...
the indexes i,j,k defined above indicate the position in the 4x4x4 grid
of points but they are given in the FFT-like convention where point 1 1
1 is the origin... so R should rather be R=(i-1)*a1+(j-1)*a2+(k-1)*a3...
where a1,a2,a3 are the Bravais  lattice vectors.
Notice also that the force constant grid is written as these were
periodic ...
so in a 4x4x4 grid R(1,1,1) = 0,  R(2,1,1,) = a1,  and  R(4,1,1)=-a1

hope this helps,

stefano de Gironcoli - SISSA and DEMOCRITOS
```