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Hi,
I am trying to perform DFT+U calculations on an ionic compound.

I run the calculations on 8 nodes and for DOS calculation, when i dont specify NPAR tag the result is 1.8eV indirect band gap. But with exactly the same input files, when i use NPAR=2 the result is 1.3eV direct band gap.

Do you know the reason of it?
Also there is a big difference between results. Which result is reliable?

Many thanks

NPAR should not change the results, please check for other differences in the two runs.

Dear Admin

I checked the input files and results twice. The input files are completely same. The only difference is the NPAR=2 tag in one of them. Although starting with same geometry and same convergence parameters, the final geometries and charge densities are different in the relaxation. So DOS calculations give different results.

NPAR settings affect the number of _empty_ bands, if I'm not terribly wrong. Maybe here is your trouble.

Cheers,

alex

Dear Alex
As far as I know, NPAR is used for parallelisation to improve the performance of the job. It should not change the results. So do you have any advice to me?

I agree, it should not change a thing. But well, you might guess that it might be more complicated in the real world. Maybe your NBANDS are simply not sufficient?

Try typing

grep NBANDS OUTCAR

for both NPAR settings and tell us the result. What is the number of your last occupied band?


Cheers,

alex

I checked the results. The number of last occupied bands in both calculations are same.

PS: Now i submitted a new calculation with increased number of bands and will see what happens.

Nothing has changed!

Based on my experience, I think NPAR isn't a purely computational setting.
For some calculations involving particular materials or systems, NPAR gives us very different physical quantities as you said. It may also causes some convergence problems in some cases.