spurious shifts in LOCPOT via LVHAR

[taishan@mit.edu]

Dear All,

I'm calculating the potential over graphene, using LVHAR, on a supercell of 10x10 conventional cells.
I was expecting a uniform field with fluctuations near C atoms. However, I'm seeing a strange potential surface like a basin: The top and bottom area has higher potential than the middle area. Even in the middle area, the left and right boundaries have slightly higher potential.

Does anyone have any thoughts about the possible reasons please? Thanks very much.

Best,
Tai

[henrique_miranda]

Hi,

Without seeing a figure or the input files its difficult to say exactly what it might be.
Maybe you could attach a figure of the potential.
Attaching the INCAR, POSCAR, KPOINTS the names of the POTCARS and OUTCAR is also useful.

Without seeing those I might guess that you are having some egg-box effect in your calculation
wiki/index.php/PREC

In case you did not set it already, try re-doing the calculation with PREC=Accurate

[taishan@mit.edu]

Dear Henrique,

Thank you very much for your reply, and the suggestion on increasing the accuracy criterion.

Here is the figure:

LOCPOT_LVHAR.png

Best,
Tai

[taishan@mit.edu]

... and here are the input files.

[henrique_miranda]

Here are a few comments:

1. The figure you show represents the value of the potential on the x and y coordinates but for which value of z? To me, it looks like you are representing z=0 while the graphene layer is in z=21.988 Angstrom
2. What are the values in the color scale? If the difference between red and green is very small then I would not worry about this 'spurious effects' too much.
3. I believe you are using a too large vacuum in the z-direction (44.5639155825 Angstrom). You might be able to reduce it substantially and speeding up the calculation a lot without greatly affecting the final results of your calculation (depending on the application you intend).
4. Did you try producing the same image using PREC=Accurate?

Hope these help you to track down the issue

[taishan@mit.edu]

Dear Henrique,

Thank you very much for helping posting and the nice comments.
1. I'm plotting the potential over a place above the graphene atoms. You're right that the graphene layer is in z0=21.988, and I'm plotting a layeer 3.3A above z0.
2. The color range is 0.6V-0.77V, and the difference is too large to be ignored.
3. You are also right that I'm using too large vacuum, which is only for debugging purpose.
4. I have tried PREC=accurate, and the results are almost identical.

Previously you suggested the egg-box effects, although my case is not the case, it looks like an egg in the box too.
Do you have any thoughts about this please? Thank you very much.

Best,
Taishan

[henrique_miranda]

1. Can you plot the value of the potential on a plane near the graphene layer?
2. You are assuming that what you see in the figure you produced are 'spurious effects' why do think so? What is the value of the potential closer to the carbon atoms? Is this not a distorted graphene structure?

[taishan@mit.edu]

Hi Henrique,
Thank you very much for helping.
1. The potential is 3.3A away, above the carbon atoms. Do you mean less distance, say 1A ?
2. I thought the potential distribution is "spurious" or unexpected, because I was expecting a more or less uniform field, due to translational symmetry. The graphene is not distorted.

[henrique_miranda]

1. Yes try plotting the potential on a plane that is close to the graphene atoms z= 21.988 +- 1. Or even better plot the isosurface.
VESTA supports reading the LOCPOT file: https://jp-minerals.org/vesta/en/

2. Check the symmetry of your system and check the symmetry of pure graphene. The POSCAR you posted is distorted.

[taishan@mit.edu]

Dear Henrique,

Thank you very much for your comments, which are extremely helpful. I checked the structure, like you said, somehow the graphene was distorted, although I didn't have any clue how that happened. I tried pristine graphene, and it works pretty much "normal" now. Thank you so much.

Best,
Tai