My Community

Hello,
I was doing computation of tetragonal phase CsPbBr3 perovskites.
After relaxing the crystal I changed to ISMEAR=-5 for computation of total energy.
As I was intended to apply strain to the crystal, the cell parameters were rounded (even for zero strain scenario).
The difference of rounded and unrounded cell parameters were rather small.

For the unrounded:

8.1735886051970006 0.0000000000000008 0.0000000000000008
0.0000000000000022 8.1735886051970006 0.0000000000000008
0.0000000000000004 0.0000000000000004 11.9295151065315821

and for the rounded:

8.173589 0.000000 0.000000
0.000000 8.173589 0.000000
0.000000 0.000000 11.929515

The rest of POSCAR and input files were totally identical.

However, the following warning appeared during computation, but only for rounded POSCAR scenario:
-----------------------------------------------------------------------------
| |
| W W AA RRRRR N N II N N GGGG !!! |
| W W A A R R NN N II NN N G G !!! |
| W W A A R R N N N II N N N G !!! |
| W WW W AAAAAA RRRRR N N N II N N N G GGG ! |
| WW WW A A R R N NN II N NN G G |
| W W A A R R N N II N N GGGG !!! |
| |
| Tetrahedron method does not include variations of the Fermi |
| occupations, so forces and stress will be inaccurate. We suggest |
| using a different smearing scheme ISMEAR = 1 or 2 for metals in |
| relaxations. |
| |
-----------------------------------------------------------------------------

And the total energy becomes lowered and deviates from normal energy-strain curve.
(-.63854333E+02 for rounded and -.63853921E+02 eV for unrounded)

This only happens to zero strain scenario, and there does not seem to be any warnings and problems for any runs with finite strain implemented.

The following is the INCAR for the static run:

CsPbBr3-Cubic
ISTART = 0
ICHARGE = 2
LCHARG = .FALSE.
LWAVE = .FALSE.
ENCUT = 750
EDIFF = 1.00000E-8
ALGO = 48
NSW = 0
IBRION = 2
ISIF = 0
POTIM = 0.4
ISMEAR = -5
ISPIN = 1
NPAR = 6
LREAL = Auto
PREC = Accurate

What I do not really understand is that, the differences between the two input structures are so small, is it likely for this to cause such a big problem? Or is it rather because I did some other things wrong during some other steps?

Hi,

I can have a look at it. How exactly are you applying the strain?
Can you please upload the input and main output files according to the forum guidelines?

Cheers,
Marie-Therese

I just mentioned that I rounded the cell parameters for a little bit for the sake of applying strain. I didn't apply strain to this one though.

The following file is the file for the run in which warning appears.

The following is the file for the static run in which no warning appears.
Their only difference is the lattice parameters in POSCAR.
By the way, this only happens to ISMEAR=-5 run.
Gaussian smearing scheme is totally fine.

By the way, this issue seems resolved by increasing KPOINTS from 8 8 6 to 8 8 8
However, when doing convergence test the energy is already well converged in 8 8 6 setting