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I'm new to DFT calculations and trying to produce the Equation of State curves (e.g. Pressure vs Volume) of Graphite. I started relaxing the system with ISIF=3 and then gradually scaled up and down the relaxed cell (with ISIF=4) in order to create the different points on the PV curve. However, my PV curve demonstrates abnormal spikes and I couldn't find out the issue. I was able to produce the PV curve for Diamond and it came out smoothly following the same procedure. Any advice is highly appreciated. I'm using PAW_PEB for carbon.

My INCAR for initial relaxation looks like this,

SYSTEM= C_graphite
PREC=  A
ENCUT= 400.0
EDIFF= 1e-6
IBRION= 2
ISIF= 3
NSW= 100
ISMEAR= 0 ; SIGMA = 0.05
ISTART=0
POTIM= 0.5
LCHARG= .FALSE.
LWAVE= .FALSE.
LREAL= .FALSE.
EDIFFG = -1e-2
ISYM=0

And POSCAR looks like this for Graphite,

C4
1.0
   1.2336456308015413   -2.1367369110836267    0.0000000000000000
   1.2336456308015413    2.1367369110836267    0.0000000000000000
   0.0000000000000000    0.0000000000000000    7.8030730000000004
C
4
direct
   0.0000000000000000    0.0000000000000000    0.2500000000000000 C
   0.0000000000000000    0.0000000000000000    0.7500000000000000 C
   0.3333333333333333    0.6666666666666666    0.2500000000000000 C
   0.6666666666666667    0.3333333333333334    0.7500000000000000 C

I have used automatic k-point mesh as follows,

EA
0
Auto
  32

My PV curve looks like this and it definitely has an error, enter image description here

Can you guys please advise me to find out my mistake?

Thank you very much.

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  • 2
    $\begingroup$ plot results from ISIF=2 instead of 4 and check $\endgroup$
    – Pranav kumar
    Jun 6 at 1:47
  • $\begingroup$ Thank you @Pranavkumar. It worked. But I'm afraid about the restrictions imposed with ISIF=2 (the shape of the unit cell is held constant). could it affect the phase transitions that might take place with the compression/expansion of the system? $\endgroup$
    – WhiteLeo9
    Jun 9 at 12:19
  • 1
    $\begingroup$ Are you expecting graphite to change in to diamond with ISIF=4? Another point ISIF=4 is putting restrictions of keeping volume to be fixed whereas ISIF=2 is not considering any degree of freedom from simulation box. Hence minimization process is much difficult using ISIF=4 $\endgroup$
    – Pranav kumar
    Jun 10 at 0:37
  • $\begingroup$ If Pranav's questions in the comments get ignored, the question can get closed as "abandoned". I'm not saying that this will happen, but it's something to consider. $\endgroup$
    – Nike Dattani
    Jun 13 at 17:46
  • 1
    $\begingroup$ Starting with certain symmetry it is difficult to achieve completely different symmetric structure. Phase transition is not possible as system is stuck in local minima or metastable structure. $\endgroup$
    – Pranav kumar
    Jun 15 at 3:28

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