[Pw_forum] IFCs and supercell

[Paolo Giannozzi]

Great. Now I can add a fourth link to point 7.7 of the FAQ,
http://www.quantum-espresso.org/faq/phonons/

P.

On Thu, 2014-11-06 at 18:54 +0100, stefano de gironcoli wrote:
> 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
> > 
> > Address:
> > 
> > EPFL STI IMX LC
> > 
> > MXD 220 (Bâtiment MXD) Station 12
> > 
> > 1015 Lausanne, Switzerland
> > 
> >  
> > 
> > 
> > 
> > 
> > _______________________________________________
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> 
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