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Hello everybody,

I wanted to calculate the chemical potential of Sn by dividing the total enery of bulk Sn by the number of atoms in the structure. My total energy turne out to be positive(TOTEN = 28.13713526 eV) and I know it should be negative. Please give me any suggestions about what I'm doing wrong.

here is my poscar:

Sn - white tin_tetragonal
5.83
1 0 0
0 1 0
0 0 0.55
8
Direct
0.00000000 0.00000000 0.00000000
0.25000000 0.75000000 0.25000000
0.00000000 0.50000000 0.00000000
0.25000000 0.25000000 0.75000000
0.50000000 0.50000000 0.50000000
0.75000000 0.25000000 0.75000000
0.50000000 0.00000000 0.50000000
0.75000000 0.75000000 0.25000000

and the INCAR:

SYSTEM = Sn white tin (tetragonal)
ISTART = 1
ICHARG = 0
ENCUT = 300
PREC = Normal

LREAL = .False.
ISMEAR = -5

I also tried with ISMEAR = 1; SIGMA = 0.1, but the resutl was the same. I used PAW-PBE Sn_d potential.

Thanks,
Zsolt

If you know that it should be negative you can start with making sure that:

1. Your geometry is correct by using some viewer. If one compares with Phys. Rev. 129 p2483 (1963) it seems it is only 4 atoms in the unitcell you define, at least at first glance. I might be wrong.
2. The lattice parameters have been optimized.
3. You have accounted for the correct magnetic structure like ferromagnetism, antiferromagnetism, etc.
4. The results are converged wrt cutoff-energy and k-point sampling.

Best regards,
/Dan Fors

<span class='smallblacktext'>[ Edited Wed Apr 23 2008, 10:52PM ]</span>

it seems that you forgot to increase the unit cell size (volume) when you increased the number of atoms, therefore the cell you calculate has enormous pressure. please do a few geometry relaxation steps
ISIF=3; IBRION=2; PREC=Accurate; ENCUT=1.3*ENMAX(POTCAR)
to first of all check the pressure on the cell you gave in POSCAR, and furtheron to obtain the equilibrium structure. This will give negative total energies.