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Hi Dear all,

I am new user of VASP and I have a problem when I do some single point energy calculations.

I have done structure optimization on organic crystals and I set EDIFFG to -0.001. And it works. But after doing a SPE calculation, when I look at the OUTCAR file, some atoms have their forces upper 0.001 eV/Ã….

I do not understand how forces can increase so far after a SPE calculation.

Thanks

Hi,
This is surprising. Have you

1. Done the SPE on relaxed geomtrey moving the CONTCAR of relaxation to POSCAR of SPE?
2. Done SPE in SCF (ICHARG = 0 or 2 or 1) or non-SCF (ICHARG = 11 or 12) fashion?
3. Changed the BZ integration scheme (for ex from Gaussian to Tetrahedron+Blöchl or MP) without IBRION = -1, although of course NSW=0 without IBRION=-1 setting would do fine?
4. Changed the K-mesh density?
5. Changed energy cutoff?

Perhaps you could quot here your INCAR fiels for relaxation and SPE.

Cheers,
Sankh

Hi,

This is the INCAR file from the relaxation:

NWRITE = 1
EMIN = -15
EMAX = 15
IALGO = 38
NELM = 30
EDIFF = 0.0001
EDIFFG = -0.001
IBRION = 1
ISMEAR = 4
SIGMA = 0.2
ENCUT = 520
ISIF = 3
ISYM = 0
NSW = 800
NELMIN = 4
LREAL = .FALSE.
PREC = High

And this is the INCAR file from SPE:

NWRITE = 1
EMIN = -15
EMAX = 15
IALGO = 38
NELM = 30
EDIFF = 0.00001
IBRION = -1
ISMEAR = 4
SIGMA = 0.2
ENCUT = 520
NELMIN = 4
LREAL = .FALSE.
PREC = High

I have used the CONTCAR file from relaxed calculation and turned it into POSCAR for SPE calculation.
And both calculations have the same K-mesh.

Thanks,
Gaëtan

<span class='smallblacktext'>[ Edited Wed Mar 09 2011, 09:40AM ]</span>

I noticed that you changed EDIFF from 1E-4 to 1E-5. Did you already check that 30 steps (NELM=30) are enough to achieve convergence for the SCF ?
Did you compare the energies computed from your optimization and your SPE ? Do they differ?
In your optimization, the symmetry is turned off (ISYM=0), whereas in the SPE not. Does VASP detect any symmetry, when you do the SPE? Check the outcar file.


Andreas

I always check if my calculation converge or not and in this case, the converge is ok.

I get -435.173052 eV for the optimization and -435.172817 eV for the SPE.

And yes, VASP detect the symmetry I expected.

Thanks,

Gaëtan

If VASP detects a symmetry in the SPE, then, the two calculations are performed under different symmetry restrictions. (Which might lead to different electronic states). Try taking a look at your occupied orbitals and check the occupation numbers.

Try setting ISYM=0 also for the SPE and then compare the energy with the optimization job.
I would also try to perform a symmetry restricted (ISYM=1 or 2) optimization and compare the energy with the ISYM=0 job.

Hope that helps.

Andreas

Hi,

So, I have tried two thnings:

- Perform a SPE with ISYM = 0 tag (E= -435.172817 eV, same as ISYM = 2 SPE)


-Perform a symmetry restricted optimization (E= -435.172821 eV and then a symmetry restricted SPE (E= -435.172752 eV).

Both attempt lead to an increase in forces for the SPE.

most probably, the following has occured:
I see that you chose ISIF=3 for the first run. This changes the shape and the volume of the cell during the run, without updating the PW-meshes.
If volume and cell shape are changed considerably, the PW-mesh used throughout the run becomes worse in quality as compared to the input geometry.
For the follow-up SPE calculation, the PW-meshes are re-calculated, in order to give the desired accuracy for ENCUT and the new cell dimensions. ---> with the change of the basis set, the forces will change as well.
please compare the NGX, NGY and NGZ parameters of runs 1 and 2 to see if this may be the reason of the inconsistency of the forces