[QE-users] [PW_forum] IFC file structure

[Lorenzo Paulatto]

q2r.x produces a list of force constant order by unit cell inside the 
super-cell, the distance between the atoms is not taken into account. 
Also, they come from a finite q-point grid calculation, hence they are 
periodic on the supercell reciprocal to the grid size.

Consider that building force constants is less trivial than you put it, 
because the positions of atoms in the unit cell is free (i.e. they could 
be outside it), and because not all cells are simple cubic, building a 
list of neighbors to a certain atom requires building a Wigner-Seitz 
(WS) cell.

This done on the fly by the matdyn code via the wsinit/wsweight codes, 
which are quite obscure but not impossible to understand if you think 
about what the WS cell is. Also, atoms could be on the border of each 
other WS cell, which requires splitting the force constant among all 
equivalent positions to maintain symmetry. I call these operations 
"centering".

However, if all you want are the centered force constants, the d3_q2r.x 
code included in the D3Q package (make d3q) does it. It takes the same 
input as q2r and produces a very similar force constant file.

kind regards


On 6/6/19 2:15 AM, Adrien Descamps wrote:
> Dear all,
> 
> I am trying to calculate the phonon dispersion for Silicon. Thanks to 
> the previous posts, I think I understand the structure of the output 
> file and how to extract the interatomic force constant matrices.
> Since the Cartesian frame is given by the vectors (-1,0,1)a/2; 
> (0,1,1)a/2 and (-1,1,0)a/2, where can I find the IFC for the interaction 
> between the two atoms in the unit cell, namely the interaction between 
> atom at (0,0,0) and the 4 nearest neighbors at (1/4,1/4,1/4), 
> (1/4,-1/4,-1/4), (-1/4,1/4,-1/4), (-1/4,-1/4,1/4)? I would expect to 
> find it in the section 1 1 1 2 section but none of the position vector 
> match the four first nearest neighbor, unless I making a mistake:
> 
> 1   1   1   2
>     1   1   1  -6.76355682813E-02
>     2   1   1   7.83560937500E-05
>     3   1   1  -4.45195312500E-05
>     4   1   1   7.20209843750E-04
>     1   2   1  -6.76355682813E-02
>     2   2   1  -4.13570781250E-04
>     3   2   1  -4.13570781250E-04
>     4   2   1  -6.76355682813E-02
>     1   3   1  -4.63088953125E-03
>     2   3   1   1.34053593750E-04
>     3   3   1  -4.13570781250E-04
>     4   3   1   7.20209843750E-04
>     1   4   1  -4.63088953125E-03
>     2   4   1  -4.13570781250E-04
>     3   4   1  -4.45195312500E-05
>     4   4   1   3.78551718750E-04
>     1   1   2   7.83560937500E-05
>     2   1   2   1.17258593750E-04
>     3   1   2   3.07267343750E-04
>     4   1   2   7.20209843750E-04
>     1   2   2  -4.13570781250E-04
>     2   2   2   1.34053593750E-04
>     3   2   2  -4.63088953125E-03
>     4   2   2   7.20209843750E-04
>     1   3   2   1.34053593750E-04
>     2   3   2   1.34053593750E-04
>     3   3   2   3.07267343750E-04
>     4   3   2   3.07267343750E-04
>     1   4   2  -4.13570781250E-04
>     2   4   2   1.17258593750E-04
>     3   4   2   2.44567968750E-04
>     4   4   2   3.07267343750E-04
>     1   1   3  -4.45195312500E-05
>     2   1   3   3.07267343750E-04
>     3   1   3   3.78551718750E-04
>     4   1   3   2.42503593750E-04
>     1   2   3  -4.13570781250E-04
>     2   2   3  -4.63088953125E-03
>     3   2   3   3.78551718750E-04
>     4   2   3  -4.45195312500E-05
>     1   3   3  -4.13570781250E-04
>     2   3   3   3.07267343750E-04
>     3   3   3   2.44567968750E-04
>     4   3   3   1.17258593750E-04
>     1   4   3  -4.45195312500E-05
>     2   4   3   2.44567968750E-04
>     3   4   3   2.44567968750E-04
>     4   4   3  -4.45195312500E-05
>     1   1   4   7.20209843750E-04
>     2   1   4   7.20209843750E-04
>     3   1   4   2.42503593750E-04
>     4   1   4   2.42503593750E-04
>     1   2   4  -6.76355682813E-02
>     2   2   4   7.20209843750E-04
>     3   2   4  -4.45195312500E-05
>     4   2   4   7.83560937500E-05
>     1   3   4   7.20209843750E-04
>     2   3   4   3.07267343750E-04
>     3   3   4   1.17258593750E-04
>     4   3   4   7.83560937500E-05
>     1   4   4   3.78551718750E-04
>     2   4   4   3.07267343750E-04
>     3   4   4  -4.45195312500E-05
>     4   4   4   2.42503593750E-04
> 
> Thank you very much,
> 
> Best regards,
> Adrien Descamps
> 
> Stanford Unversity, Aeronautics and Aerospace Engineering
> 
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-- 
Lorenzo Paulatto - Paris