[Pw_forum] IFCs and supercell

[Kvasov Alexander]

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
Address:
EPFL STI IMX LC
MXD 220 (Bâtiment MXD) Station 12
1015 Lausanne, Switzerland

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