[Pw_forum] Force constants

[Iyad AL-QASIR]

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,
IYAD Al-Qasir

X-ray and Neutron Scattering and Spectroscopy Group
Materials Science and Technology Division
ORNL
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