[QE-users] Incorrect heat of formation obtained for FeSi(NaCl structure) relaxation

[Kefan Chen]

Hi there,

I'm using QE 6.5 to run a relaxation simulation with FeSi(NaCl structure)
supercell. I put 8 atoms in the cell and I used simple cubic for crystal
symmetry. Here's the input script I used:

&control
calculation='vc-relax',
prefix='FeSi_vc-relax_fixed'
pseudo_dir='/home/kchen/PSEUDO/'
/
&system
ibrav=1,
celldm(1)=9.032891,
nat=8,ntyp=2,
ecutwfc=100,
ecutrho=1500,
occupations='fixed'
/
&electrons
mixing_beta=0.3,
electron_maxstep=300
/
&ions
/
&cell
cell_dynamics='bfgs'
/
ATOMIC_SPECIES
Fe 55.845 Fe.pbe-spn-rrkjus_psl.1.0.0.UPF
Si 28.0855 Si.pbe-n-rrkjus_psl.1.0.0.UPF
ATOMIC_POSITIONS (crystal)
Fe 0  0  0
Si  0.5  0  0
Fe 0.5  0.5  0
Si  0  0.5  0
Fe 0.5  0  0.5
Si  0  0  0.5
Fe 0  0.5  0.5
Si  0.5  0.5  0.5
K_POINTS (automatic)
6 6 6 1 1 1

I got the final Fe-Si bond length as 2.33 A and Heat of Formation as
0.04eV/atom. However, in the reference,
https://journals.aps.org/prb/pdf/10.1103/PhysRevB.85.245102, page 6, Table
VI, the bond length is around 2.37A, which is close to my result but the
heat of formation is 0.25 eV/atom. I don't know what I'm doing wrong. For
potential energy of Fe and Si. I used 1 Fe atom in bcc crystal and 2 Si
atoms in fcc crystal for their potential energies. You can find the script
I used in the attachment. The potential energy for bcc Fe and dia Si I got
are -247.789Ry/atom and -11.4201Ry/atom respectively.

I also tried SCF calculation with the bond length provided in the
reference, but it did not help a lot.

Thank you for your help

Kefan

Ph.D. student
Department of Chemical and Materials Engineering
University of Nevada, Reno, USA
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