Energy differences among different unit cells of the same structure

[Fm3m]

I'm doing simulations of a Pm-3m cubic perovskite (#221). I have 3 different cells/supercells of the same structure made with different lattice vectors. POSCARs of these cells are given at the bottom of this comment. I have two problems. The first is that while I know for a fact that these are all valid unit cells for my structure (tile 3D space to a structure of space group Pm-3m), they come out with different total energies per formula unit. The total energies differ by about 1 meV/f.u. Second, VASP correctly recognizes the symmetry in simulations #1 and #2 as O_h, but in structure #3 VASP gives a symmetry of D_4h.

Does anyone have any advice?

Thank you.

POSCAR #1
CsSnF3
4.69652595
1 0 0
0 1 0
0 0 1
Cs Sn F
1 1 3
Direct
0 0 0
0.5 0.5 0.5
0.5 0.5 0
0.5 0 0.5
0 0.5 0.5

POSCAR #2
CsSnF3
4.69652595
1 1 0
0 1 1
1 0 1
Cs Sn F
2 2 6
Direct
0 0 0
0.5 0.5 0.5
0.25 0.25 0.25
0.75 0.75 0.75
0.5 0 0
1 0.5 0.5
0 0.5 0
0.5 1 0.5
0 0 0.5
0.5 0.5 1

POSCAR #3
CsSnF3
4.69652595
1 -1 0
1 1 0
0 0 1
Cs Sn F
2 2 6
Direct
0 0 0
-0.5000 0.5000 0
0 0.5000 0.5000
-0.5000 1.0000 0.5000
0 0.5000 0
-0.5000 1.0000 0
0.2500 0.2500 0.5000
-0.2500 0.7500 0.5000
-0.2500 0.2500 0.5000
-0.7500 0.7500 0.5000

[alex]

Hi Fm3m,

could you please post the INCAR and the different KPOINT-files?

Thx,

alex

[Fm3m]

INCAR (same for all structures):

ISMEAR=-5

LWAVE = .FALSE.
LCHARG = .FALSE.

PREC= Accurate
ENCUT = 600
EDIFF=1e-6
NELMIN=6
GGA=PS

ISIF=7
IBRION=2
EDIFFG=-0.0025
POTIM=0.25

[Fm3m]

KPOINT:

Automatic mesh
0
Gamma
6 6 6
0 0 0

[alex]

Hi Fm3m,

you ended up with qualitatively different k-meshes for your (different) cells and cell sizes. Check the OUTCAR for the actual meshes and you'll see.

Cheers,

alex