VASP calculation of cd Si

Series of try VASP sample. “diamond Si” here.

http://cms.mpi.univie.ac.at/wiki/index.php/VASP_workshop_2003
http://cms.mpi.univie.ac.at/wiki/index.php/Cd_Si

Calculation is the same as that of fcc Si. So, I don’t explain the input file.

Results

The result is the following.

The horizontal line shows the lattice constant and the vertical line shows Etot. I think Etot was estimated for number of atoms defined in POSCAR. In this case, I divided the estimated energy by 2. It means the vertical lines shows “eV/atom”.

Relaxation

Moreover, I tried the sample on relaxation (cd Si volume relaxation), and I write down the results here.

The input files were explained in fcc Si, the new tags were appeared.

INCAR

System = diamond Si
ISMEAR = 0
SIGMA  = 0.1
ENMAX  =  240
IBRION = 2 
ISIF   = 3
NSW    = 15
EDIFF  = 0.1E-04
EDIFFG = -0.01

EDIFF specifies the global break condition for the electronic SC-loop. The default value is 1.0E-4.

EDIFFG defines the break condition for the ionic relaxation loop. If the change in the total (free) energy is smaller than EDIFFG between two ionic steps relaxation will be stopped. If EDIFFG is negative it has a different meaning: In this case the relaxation will stop if all forces are smaller than |EDIFFG| . This is usually a more convenient setting. The default value is EDIFF * 10.

Results

The most stable lattice constant was 5.465 Å and the energy was E0 = -5.414. The graph was the following.

Comparing with fcc Si, the lowest energies are:
fcc Si: -4.87 eV/atom
diamond Si: -5.414 eV/atom

So, diamond Si is more stable. This is the results in 0K and 0 Pa. I don’t know the case of R.T. and 1 atm.

DOS

The result of DOS calculated with the result of relaxation is the following.

INCAR is the same as fcc Si DOS. DOSCAR calculated by relaxation is no meaning. If we wan to get the precise result, we need to perform the static calculation (ISTART=1; NSW=0;) with CONTCAR (changing the name to POSCAR) after relaxation.

The horizontal line is E=EF(eV) and the vertical line is DOS. Around E = 0, we can see the band gap. It means fcc Si is semiconductor. But, the value of band gap is not precise. The band figure is the following (INCAR is the same as fcc Si bandstructure).

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