[QE-users] Calculating formation energy of the defect

[mehrdad zamzamian]

Hi all dear QE users
I'm working on the formation energy of a single Fe vacancy in Fe3C
(cementite). It seems that we can calculate this parameter by:
Eform=EFe31−xCtot–EFe3Ctot+x⋅EFetot,
where the first term is the total energy of Fe3C with a Fe vacancy, the
second one is the total energy of Fe3C (without defect) and the 3rd one is
the total energy of a single Fe. x for this case can be 1 because we have a
Fe vacancy. Is this a correct way to calculate?
the second problem is the scf calculation does not converge and begins to
fail after almost 75 iterations (see figure) for the case of pure Fe (when
i want to calculate Fe-vacancy in BCC-Fe). These conditions also exist for
optimization, i can reach neither to convergence for scf nor to an
optimized geometry. Here is my input.

[image: BCC-Fe3.png]
&CONTROL
    calculation = "scf"
    max_seconds =  8.64000e+04
    pseudo_dir  = "C:\Users\pseudo"
/&SYSTEM
    a                         =  5.03018e+00
    b                         =  6.73931e+00
    c                         =  4.49084e+00
    degauss                   =  1.00000e-02
    ecutrho                   =  2.25000e+02
    ecutwfc                   =  2.50000e+01
    ibrav                     = 8
    nat                       = 16
    nspin                     = 2
    ntyp                      = 2
    occupations               = "smearing"
    smearing                  = "gaussian"
    starting_magnetization(1) =  2.00000e-01
    starting_magnetization(2) =  0.00000e+00
/

&ELECTRONS
    conv_thr         =  1.00000e-06
    electron_maxstep = 200
    mixing_beta      =  4.00000e-01
    startingpot      = "atomic"
    startingwfc      = "atomic+random"
/

K_POINTS {automatic}
 2  2  3  0 0 0

ATOMIC_SPECIES
Fe     55.84500  Fe.pbe-nd-rrkjus.UPF
C      12.01070  C.pbe-rrkjus.UPF

ATOMIC_POSITIONS {angstrom}
Fe      0.892203   0.460793   1.491414
Fe      4.137977   6.278514   2.999431
Fe      1.622887   6.278514   3.736836
Fe      3.407293   0.460793   0.754008
Fe      3.407293   2.908860   0.754008
Fe      1.622887   3.830447   3.736836
Fe      4.137977   3.830447   2.999431
Fe      0.892203   2.908860   1.491414
Fe      0.184411   1.684827   3.759376
Fe      4.845769   5.054480   0.731469
Fe      2.330679   5.054480   1.513954
Fe      2.699502   1.684827   2.976891
C       0.618742   5.054480   2.519058
C       4.411438   1.684827   1.971786
C       1.896348   1.684827   0.273636
C       3.133833   5.054480   4.217209


Thanks in advance
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