Re: [Pw_forum] IFCs and supercell

```dear Alexander,

```
the grid correspond to a 8x8x8 supercell (or rather to the FFT of a 8x8x8 q-point grid ... which is the same)
```
```
atomic pairs are specified by the indices of the two atoms in the unit cell plus the direct lattice vector connecting the two cells in which they are located with two important details to be considered
```    1) the origin is the 1,1,1 element of the list
```
2) the IFC are obtained by a FFT from the dynamical matrices of the q-point grid and are therefore written assuming 8 8 8 periodicity (in the present case) the corresponding relevant real space vectors vectors are therefore given by r(i) = (n1-1)*at(i,1)+(n2-1)*at(i,2)+(n3-1)*at(i,3) + their periodic refolding to points nearest the origin
```a few examples should clarify:
1 1 1  -> r =    0
2 1 1  -> r =  a1
8 1 1  -> r = -a1

```
the routine weight play the fundamental role of trimming the vector in such a way the only the points inside the wigner-seitze
```cell of the supercell contribute
weight (r) = 1 if r is inside the WS cell
weight (r) = 0 if r is ouside the WS cell
```
weight (r) = 1/(n+1) if r is on the surface of the WS cell where n is the number of planes that pass through it
```
stefano

On 11/06/2014 06:14 PM, Kvasov Alexander wrote:
```
```
Dear All,

```
My question is: how the supercell (which is used by q2r and matdyn) is specified? If anybody has any information about this I would highly appreciate.
```
```
“fltau” option of matdyn gives atomic positions only for 1 cell and not for N1xN2xN3 supercell.
```
```
Apparently, there are some tricks… for example, here one has some IFCs for 8x8x8 grid of SrTiO3
```
...

8   8   8

1   1   1   1

1   1   1   5.72142674655E-02

2   1   1  -1.06343912107E-02

3   1   1   1.01649714042E-04

4   1   1   4.15611567060E-06

5   1   1  -6.46046533014E-06

6   1   1   4.15611567060E-06

7   1   1   1.01649714042E-04

8   1   1  -1.06343912107E-02

...

```
1. first, IFC values decries with N1 and then increase, so they have to be shuffled
```
```
2. second, it seems SUBROUTINE setupmat and frc_blk is the right place to look for this
```
…

DO na=1, nat

DO nb=1, nat

total_weight=0.0d0

DO n1=-2*nr1,2*nr1

DO n2=-2*nr2,2*nr2

DO n3=-2*nr3,2*nr3

!

! SUM OVER R VECTORS IN THE SUPERCELL - VERY VERY SAFE RANGE!

!

DO i=1, 3

r(i) = n1*at(i,1)+n2*at(i,2)+n3*at(i,3)

END DO

…

but summation goes from -2N to 2N, which is not clear for me.

3. what is “weight”  in SUBROUTINE setupmat and frc_blk

If anybody could explain what is going on here it would be great!

Sincerely yours,

Alexander Kvasov

Scientist

Ceramics Laboratory (LC)

École Polytechnique Fédérale de Lausanne (EPFL)

Phone: +41 21 693 5805

EPFL STI IMX LC

MXD 220 (Bâtiment MXD) Station 12

1015 Lausanne, Switzerland

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