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Hello all,

I am calculating the energy of a supercell for BCC iron. The cell contains 53 atoms and 1 vacancy + 1 Oxygen atom in the center. (54 Fe atoms for a perfect structure)

Curiously, I tested the case twice with exactly the same input. But the output energy is totally different. The difference is ～2eV.

I checked the OUTCAR in each case. The difference starts at the 6th electronic step of the 1st ionic relaxation as show in the following lines highlighted in red, where the "eigenvalue-minimisations" take distinct values. The eventual energy is -439.36743302 for case1 and -442.66293250 for case2.

Does anyone have an idea of what is wrong with the set-up. Will changing "IALGO" tag be helpful?

I sincerely thank your comments and advice.

OUTCAR for case1

(........ other outputs）
free energy TOTEN = -495.71681368 eV
energy without entropy = -495.35960210 energy(sigma->0) = -495.59774315
--------------------------------------------------------------------------------------------------------
----------------------------------------- Iteration 1( 6) ---------------------------------------
(......... other outputs)

eigenvalue-minimisations : 1153
total energy-change (2. order) : 0.2370052E+02 (-0.9220940E+02)
number of electron 429.9998602 magnetization 55.4263637
augmentation part 226.4847449 magnetization 41.1471164

(......... other outputs)

---------------------------------------------------
free energy TOTEN = -472.01629764 eV
energy without entropy = -472.13387793 energy(sigma->0) = -472.05549107
--------------------------------------------------------------------------------------------------------
(......... other outputs)

OUTCAR for case2
(.......... other outputs)
---------------------------------------------------
free energy TOTEN = -495.71681368 eV
energy without entropy = -495.35960210 energy(sigma->0) = -495.59774315
--------------------------------------------------------------------------------------------------------

----------------------------------------- Iteration 1( 6) ---------------------------------------

(......... other outputs)

eigenvalue-minimisations : 1576
total energy-change (2. order) : 0.1981334E+02 (-0.9401893E+02)
number of electron 429.9998587 magnetization 55.1799264
augmentation part 231.6494677 magnetization 40.7092657

(......... other outputs)

---------------------------------------------------
free energy TOTEN = -475.90347338 eV
energy without entropy = -476.01311241 energy(sigma->0) = -475.94001972
--------------------------------------------------------------------------------------------------------
(......... other outputs)

INCAR
System = BCC_Fe

LREAL=.FALSE.
LWAVE= .False.
#ICHARG= 1

#ISTART= 1
ISMEAR = 1
SIGMA= 0.2
#EDIFF= 1e-6
#POTIM = 0.1


#SYMPREC = 1e-3
ISIF=2
IBRION= 2
NSW= 60
#NFREE = 2

ISPIN= 2

NPAR = 4
#IALGO = 48
#LPLANE = .True.
LCHARG=.TRUE.

ENCUT= 550 eV
PREC=HIGH

KPOINTS
Automatic mesh
0 ! number of k-points = 0 ->automatic generation scheme
Monkhorst-Pack ! select Monkhorst-Pack (first letter is significant)
1 1 1 ! size of mesh (3x3x3 points along b1, b3, b3)
0. 0. 0. ! shift of the k-mesh

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