[QE-users] Format of the interatomic force constant .fc file

[Lorenzo Paulatto]

On 05/05/2025 12:52, Chirantan Pramanik wrote:
> Dear All,
>
> I am stuck with the format of the interatomic force constant file 
> obtained by the q2r.x code. Below is a preview of the .fc file. Please 
> help me to understand the entries. I need the interatomic force 
> constant, suppose, only for the atom '205'.

Hello,

please note that force constants always act between two atoms, assuming 
you know what you are doing, here is the subroutine that reads the force 
constants stripped of all the parallel distribution and ssafety checks, 
it is only a couple dozen liens of code. The subroutine reads the force 
constants for a supercell of size nr1×nr2×nr3, because of translational 
invariance, the first atom (i,na) is always in the first unit cell (i.e. 
that with coordinates 0,0,0) while the second atom (j,nb) is in cell 
(nr1-1)*a1 + (nr2-1)*a2 + (nr3-1)*a3. (Or maybe it is the second atom 
which is in cell 0,0,0, I never remember).

Because in your case the cell is 1×1×1, it won't make any difference.


hth




!-----------------------------------------------------------------------
SUBROUTINEreadfc( flfrc, nr1, nr2, nr3, epsil, nat, &
ibrav, alat, at, ntyp, amass, omega, &
has_zstar, alph, read_lr)
!-----------------------------------------------------------------------
!
IMPLICITNONE
! I/O variable
CHARACTER(LEN=256) ::flfrc
CHARACTER(LEN=80) ::line
INTEGER::ibrav, nr1,nr2,nr3,nat, ntyp
REAL(DP) ::alat, at(3,3), epsil(3,3), alph
LOGICAL::has_zstar, read_lr
! local variables
INTEGER::i, j, na, nb, m1,m2,m3
INTEGER::ios, ibid, jbid, nabid, nbbid, m1bid,m2bid,m3bid
REAL(DP) ::amass(ntyp), amass_from_file, omega
INTEGER::nt
!
OPEN(unit=1,file=flfrc,status='old',form='formatted')
!
! Read cell parameters
READ(1,*) ntyp,nat,ibrav,(celldm(i),i=1,6)
if(ibrav==0) then
read(1,*) ((at(i,j),i=1,3),j=1,3)
end if
!
CALLlatgen(ibrav,celldm,at(1,1),at(1,2),at(1,3),omega)
alat =celldm(1)
at =at / alat ! bring at in units of alat
CALLvolume(alat,at(1,1),at(1,2),at(1,3),omega)
!
! read atomic types, positions and masses
!
ALLOCATE(atm(ntyp))
DOnt =1,ntyp
READ(1,*) i,atm(nt),amass_from_file ! type and mass
END DO
!
ALLOCATE(tau(3,nat), ityp(nat), zeu(3,3,nat))
!
DOna=1,nat
READ(1,*) i,ityp(na),(tau(j,na),j=1,3) ! positions of atoms in the cell
END DO
!
! read macroscopic variables
! (dielectric constant and effective charges)
!
alph =1.0_dp
READ(1,'(a)') line
READ(line,*,iostat=ios) has_zstar, alph
!
IF(has_zstar) THEN
READ(1,*) ((epsil(i,j),j=1,3),i=1,3)
DOna=1,nat
READ(1,*)
READ(1,*) ((zeu(i,j,na),j=1,3),i=1,3)
END DO
ELSE
zeu (:,:,:) =0.d0
epsil(:,:) =0.d0
END IF
!
READ(1,*) nr1,nr2,nr3
!
! read real-space interatomic force constants
!
ALLOCATE( frc(nr1,nr2,nr3,3,3,nat,nat) )
frc(:,:,:,:,:,:,:) =0.d0
ALLOCATE( frc_lr(nr1,nr2,nr3,3,3,nat,nat) )
frc_lr(:,:,:,:,:,:,:) =0.d0
DOi=1,3
DOj=1,3
DOna=1,nat
DOnb=1,nat
READ(1,*) ibid, jbid, nabid, nbbid
READ(1,*) (((m1bid, m2bid, m3bid, &
frc(m1,m2,m3,i,j,na,nb), &
m1=1,nr1),m2=1,nr2),m3=1,nr3)
END IF
END DO
END DO
END DO
END DO
!
CLOSE(unit=1)
!
RETURN
END SUBROUTINEreadfc



-- 
Dr. Lorenzo Paulatto
IdR @ IMPMC - CNRS UMR 7590 & Sorbonne Université
phone: +33 (0)1 442 79822 / skype: paulatz
http://people.impmc.fr/lpaulatto/ - https://anharmonic.github.io/
23-24/423 B115, 4 place Jussieu 75252 Paris CX 05
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