# [Pw_forum] *.fc file in PW_SCF help.

Eyvaz Isaev eyvaz_isaev at yahoo.com
Wed Sep 17 00:32:23 CEST 2008

Dear Zhen,

--- On Thu, 9/11/08, Zhen Huang <huang87 at purdue.edu> wrote:
>
> Can anyone help me explain the real space force constant
> file *.fc. What does each index mean for example (I am > doing a 4 by4 by4 calculation): What is the second row
> 1 1 1 1 means what is saying 1.45522e+00 is this
> force constant in ev/A^2?

>
>    4   4   4
>    1   1   1   1
>    1   1   1   1.45522064500E+00
>    2   1   1  -6.48059987500E-02
>    3   1   1   1.15693187500E-03
>    4   1   1  -6.48059987500E-02
.................................
>    1   1   2  -2.61743750000E-04
>    2   1   2  -3.88856250000E-05
>    3   1   2   1.02087500000E-04
>    4   1   2  -5.68562500000E-05

FC is not a PW_SCF file, it is generated by q2r.x program, so, it is rather PW_PH file.

Well, let us to recall the notation for the force constants matrix, F_{\alpha,\beta}^{i,j}(R), where \alpha, \beta are atomic numbers, i,j are polarization vectors (x,y,z), and R is the distance between two atoms in units of lattice vectors.
Below there are examples how these lines should be treated.

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.

Hope this helps.

Bests,
Eyvaz.

-------------------------------------------------------------------
Prof. Eyvaz Isaev,
Theoretical Physics Department, Moscow State Institute of Steel & Alloys, Russia,
Department of Physics, Chemistry, and Biology (IFM), Linkoping University, Sweden
Condensed Matter Theory Group, Uppsala University, Sweden
Eyvaz.Isaev at fysik.uu.se, isaev at ifm.liu.se, eyvaz_isaev at yahoo.com