vasp gives positive Fermi energy and eigenvalues for ZnO (wurtzeit structrue).

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cyc99 · #1 Post by **cyc99** » Sun Sep 13, 2009 7:39 pm

When I calculated bulk ZnO (wurtzeit structure) with PAW or US-PP, it gave me several positive eigenvalues with positive Fermi energy (~1.5 eV).

I used Ecut=500eV and 9*9*6 k-points sampling, with which the total energy was well converged.

Also, I used experimental lattice constants of a=b=3.25A and c=5.2A.

what is the problem?

alex · #2 Post by **alex** » Mon Sep 14, 2009 2:47 pm

You deal with a 3D periodic system, so you have no vacuum around and hence the absolute fermi energy is meaningless.

Alex

panda · #3 Post by **panda** » Wed Sep 16, 2009 2:13 pm

This question has been answered in another thread related to band structure: http://cms.mpi.univie.ac.at/vasp-forum/ ... php?4.3750

From the information you have given it is impossible to determine which step in the calculation you are on. If you are still in the geometry optimization/force minimization stage then the DOS and Ef is meaningless. Only when your structure is relaxed to ground state should you obtain Ef, DOS, and band structure.

If you post your INCAR maybe someone can help you further if you still have questions.

cyc99 · #4 Post by **cyc99** » Fri Sep 18, 2009 5:44 pm

Thank you for your kind explanations.
However, still I have some questions.

1) The absolute value of the eigenvalues (AVE) also contributes to the total energy of the given system. If the AVE for 3D periodic system is meaningless, how can I calculate the surface energy which can be obtained from the energy difference between slab and bulk systems?

2) You can find my INCAR file for ZnO bulk calculation below.
Please check and let me know if there are problematic settings.

SYSTEM = ZnO_bulk

Startparameter for this Run:
NWRITE = 2; LPETIM=F write-flag & timer
ISTART = 0 job : 0-new 1-constEcut 2-costBS
IWAVPR = 1 prediction: 0-non 1-charg 2-wave 3-comb
ICHARG = 2

Electronic Relaxation 1
NELM = 30;
EDIFF = 1E-05 stopping-criterion for ELM
BMIX = 3.00
ENCUT = 500
GGA = PE

Ionic Relaxation
NSW = 200 number of steps for IOM
NBLOCK = 1; KBLOCK = 10 inner block; outer block
IBRION = 2 ionic relax: 0-MD 1-quasi-New 2-CG
ISIF = 0 stress and relaxation
ISYM = 1
SYMPREC = 1e-6
LCORR = T Harris-correction to forces
EDIFFG = -0.02

POTIM = 0.20 time-step for ion-motion
SMASS = 3.00 Nose mass-parameter (am)

DOS related values:
ISPIN = 1
ISMEAR = 0; SIGMA = 0.05 broadening in eV -4-tet -1-fermi 0-gaus

Electronic Relaxation 2
IALGO = 48 algorithm
LDIAG = T sub-space diagonalisation
LREAL = F real-space projection
NBANDS = 30

Modey3 · #5 Post by **Modey3** » Fri Oct 30, 2009 7:22 pm

"
1) The absolute value of the eigenvalues (AVE) also contributes to the total energy of the given system. If the AVE for 3D periodic system is meaningless, how can I calculate the surface energy which can be obtained from the energy difference between slab and bulk systems?"

The total energy of the system is taken with respect to the reference state used to generate the pseudopotential of each atomic constituent. Take Si for example, the pseudopotential is generated for an isolated Si atom in some sort of reference state (not necessarily the ground state!). The reference configuration energy (energy/atom) in itself is meaningless, but when you put Si into the diamond structure the energy changes with respect to the reference configuration energy. This would be the heat of formation of the Si-diamond configuration with respect to the reference configuration.

modey3