Energy differences among different unit cells of the same structure
Posted: Tue Jan 17, 2012 11:15 pm
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
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