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

[Chirantan Pramanik]

Dear Dr. Lorenzo,

Thank you for your reply and the code. I am not familiar with Fortran. I need to calculate the total force constant of the atom numbered 205 (Mg). I can understand that each pair of atoms has a force constant matrix with 9 elements, and for my case, for one supercell, there are a total of {(205*2)*9}=3690 values of matrix elements only related to the 205th  atom. I need the addition of all of these from each pair of root mean square values. Is there any direct way or any calculation in QE itself to find this?

Thanks,
Chirantan

Chirantan Pramanik
Postdoctoral Researcher
Dept. of Earth and Planetary Sciences
Weizmann Institute of Science
Rehovot, Israel
________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of Lorenzo Paulatto <lorenzo.paulatto at cnrs.fr>
Sent: Tuesday, May 6, 2025 11:09 AM
To: users at lists.quantum-espresso.org <users at lists.quantum-espresso.org>
Subject: Re: [QE-users] Format of the interatomic force constant .fc file

Caution: External Sender. Do not click on links or open attachments unless you recognize the sender.



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




!-----------------------------------------------------------------------
SUBROUTINE readfc ( flfrc, nr1, nr2, nr3, epsil, nat, &
ibrav, alat, at, ntyp, amass, omega, &
has_zstar, alph, read_lr )
!-----------------------------------------------------------------------
!
IMPLICIT NONE
! 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
!
CALL latgen(ibrav,celldm,at(1,1),at(1,2),at(1,3),omega)
alat = celldm(1)
at = at / alat ! bring at in units of alat
CALL volume(alat,at(1,1),at(1,2),at(1,3),omega)
!
! read atomic types, positions and masses
!
ALLOCATE (atm(ntyp))
DO nt = 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))
!
DO na=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)
DO na=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
DO i=1,3
DO j=1,3
DO na=1,nat
DO nb=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 SUBROUTINE readfc


--
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20250511/d118f5d3/attachment.html>