My Community

Hi,

I'm a VASP newby and I have a problem. I'm doing some GGA calculation for Au bulk. First of all I calculated the EOS. After that I varied ENCUT from 150 to 900 eV. Taking 600 eV I took the Monkhorst-Pack (upper line from 4 to 20 even and 5 to 19 odd, while in the lower line all was 0) for convergence with respect to the kpoints.

My problem is, that everything converges but from the wrong side. I thought the total energy gets minimized, but in my calculation it gets maximized.

Has anyone an idea what the cause of the problem might be?

Thanks for the replies.

<span class='smallblacktext'>[ Edited ]</span>

concerning the increase of the total energy when the
-- cutoffs are increased: if the effect is small, it may be due to the kinetic energy corrections added (please have a look at the accuracy.pdf file of the vasp-workshop)
-- k-point meshes are increased (at constant ENCUT): this really sounds strange,
please check wheter really all input parameters (except the number of k-points) were constant in these tests

Thank you,

I've repeated the calculations and in case of the cutoff I started with a reaaly small cutoff energy. So if I watched coarse enough, it all converged from the right side. It seems to be the same with the kpoints. In the mean time I have asked some experienced VASP user, who told me that small deviations can occur but are not important as long as there is convergence at all.

Hi there,

I am also a sort of a vasp newby and have a similar problem when trying to do single-point energy convergence calculations of a mineral structure (muscovite composed of Al, H, K, O and Si corresponding to a total of 84 atoms) with respect to ENCUT.


I am using:
*VASP 4.6;
*PAW_GGA Al 05Jan2001, PAW_GGA H 07Jul1998, PAW_GGA K_sv 04May1998, PAW_GGA O 05Jan2001 and PAW_GGA Si 05Jan2001;
*KPOINT file:
Automatic mesh
0
M
1 1 1
0 0 0

*INCAR file given for me to use was:
SYSTEM = muscovite-2M1

Startparameter for this Run:
NWRITE = 2
LPETIM=F write-flag & timer
ISTART = 0 job : 0-new 1-cont 2-samecut

Electronic Relaxation 1
PREC = HIGH
ENMAX = 200.00 eV
NELM = 60
EDIFF = 1E-05 stopping-criterion for ELM
BMIX = 3.00
INIWAV = 1 random initial wavefunctions
NPAR = 2

Ionic Relaxation
EDIFFG = -0.01 stopping-criterion for IOM
NSW = 0 number of steps for IOM
NBLOCK = 1; KBLOCK = 40 inner block; outer block
IBRION = 2 ionic relax: 0-MD 1-quasi-New 2-CG
ISIF = 2 stress and relaxation
IWAVPR = 1 prediction: 0-non 1-charg 2-wave 3-comb
ISYM = 1 0-nonsym 1-usesym
LCORR = F Harris-correction to forces
ISMEAR = 0; SIGMA = 0.01


From these files (conditions) and just varying the Ecut the Energies obtained were:
Ecut F= (energy in eV, correct?)
200 -542.6230600
300 -549.8089800
400 -542.2314600
500 -541.1163100
600 -541.3176400

I thought that you should always get a lower energy as you improve your basis-set, or am I doing/saying something stupid :S ?

Thanks in advance for your time,


David

Hi there,

in VASP there is some correction for (low) cutoff energies. You'll find it by
grep "kinetic energy error for atom=" OUTCAR
Sum up and remove from the total energy. The correction should be larger for small cutoffs.
The pure total energies should show the expected behaviour.

cheers,

alex

Hi Alex,

But if there is a greater correction, I checked my results and you are right about this, for smaller cutoffs then the behaviour will be augmented, no?

In my case this was translated in and logarithmic-like increase :S

ENCUT F
200 -551.0760600
300 -551.0665800
400 -542.3617600
500 -541.1432100
600 -541.3358400

Cheers


David

Hi there,

It appears that my thread was blocked while the admin was confirming my license affiliation. But I am back now so I would like to ask if anybody could please get back to me on this

Cheers


David
<span class='smallblacktext'>[ Edited Thu Feb 18 2010, 03:40PM ]</span>

Can anybody please help me with this problem, as I am stuck with this and this influences every result that I can ever get and have now started to doubt if there is any problem with my VASP :S