# [Pw_forum] Force constants

Dhruv Singh dhruv.singh at gmail.com
Sat May 5 00:24:36 CEST 2012

```Please check this
http://www.democritos.it/pipermail/pw_forum/2005-April/002408.html

On Fri, May 4, 2012 at 2:26 PM, Iyad AL-QASIR <iyad.ne at gmail.com> wrote:
> Dear All,
> I am trying to understand the format of the force constant file (*.fc)
> generated by running the executable q2r.x
>
> I hope someone could answer (or comment on) my questions below.
>
> Below is the part of q2r.f90 that prints force constants to *.fc
>
>  DO j1=1,3
>         DO j2=1,3
>            DO na1=1,nat
>               DO na2=1,nat
>                  WRITE (2,'(4i4)') j1,j2,na1,na2
>                  nn=0
>                  DO m3=1,nr3
>                     DO m2=1,nr2
>                        DO m1=1,nr1
>                           nn=nn+1
>                           WRITE (2,'(3i4,2x,1pe18.11)')   &
>                                m1,m2,m3,
> DBLE(phid(nn,j1,j2,na1,na2))             ! why nn not
>                        END DO
>                     END DO
>                  END DO
>               END DO
>            END DO
>         END DO
>      END DO
>
> Here are my notes and questions:
> 1- j1 and j2 stands for x, y, and z directions?
> 2- m1, m2, and m3 represents the integers for the position vector of a given
> unitcell?
> 3- Why in phid(nn, j1,j2,na1, na2) m1, m2 and m3 are mapped into nn? Where
> nn=m1*m2*m3
> 4- How can I build from the  *.fc file a square force constants matrix where
> each element is 3x3. That is, to build the conventional matrix we see in
> text books?
>
> Thank you very much,
>
> X-ray and Neutron Scattering and Spectroscopy Group
> Materials Science and Technology Division
> ORNL
>
>
>
>
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--
Dhruv

```