I am trying to apply electric field to a slab model in vasp. I find the DIPOL tag in the INCAR is essential to the convergence of electronic step.
According to the manual, vasp can determine the DIPOL if this tag is not user defined. However, the algorithm seems not favorable. I checked the OUTCAR and found the DIPOL determined by VASP always varied which lead a difficult convergence of energy.
On the other hand, I defined the DIPOL as the center of the cell (DIPOL = 0.5 0.5 0.5) the energy converged quickly and geometry relaxation finished successfully. However, the result shows that a very large electric field (0.7 ev/A) influences little to the electronic structure which seems quite unreasonable.
According to the manual, the position of dipole seems to have two answers. One is to set DIPOL as the center of the cell, the other is to set DIPOL as the mass center of the model.
Here is my question.
1. What is the proper setting of DIPOL under the external electric field?
2. Because this tag is used for a correction, does different setting of this tag mean a different final result?
I'm looking forward to the replays. Thank you very much!
Hi,
In VASP there are several levels of dipole correction to apply. You can use the IDIPOL tag to turn it on, and specify which direction to apply it in (1=x, 2=y, 3=z, 4=(x,y,z)). This simply corrects the total energy and forces. It does not change the contents of LOCPOT. For that, you have to also set the LDIPOL and DIPOL tags. It is not efficient to set all three at the same time for some reason. The VASP manual recommends you first set IDIPOL to get a converged electronic structure, and then set LDIPOL to True, and set the center of electron density in DIPOL. That makes these calculations a multistep process, because we must run a calculation, analyze the charge density to get the center of charge, and then run a second calculation. I think it might be the best setting.
Best,
A.
Thank you Dr. Pidatella for the kind comment!! Now, I have vesta and vtstscripts to deal with the results, but I can't find a tool to locate the center of electron density. Do you have some suggestions?
Thank you !!
Zhu
Hi Zhu,
is it not possible to get this information approximately by the charge plot in VESTA? I mean, if you center the the "center of charge" and click on it, doesn't VESTA provide the coordinate of that point? In this way you can have an approximate idea of the spatial region where it is located.
Let me know.
Cheers,
A.
Hi Dr. Pidatella,
It seems that VESTA can only visualize the charge density, but hard to deal with the Data. I mean calculating the center of charge seems not available in VESTA. Do you have another solution？
Thank you for the kind help!!
Zhu
Hi Zhu,
Have you tried to visualize the corresponding CHGCAR file with VMD?
http://www.ks.uiuc.edu/Development/Download/download.cgi?PackageName=VMD
Upload the file with VMD and then:
Graphics-> Representations -> Drawing Method -> Isosurface
Hope it helps!
Best,
A.
P.S.: I'm attaching a link for getting a code for the Bader analysis (concerning the charge density analysis). It can open the CHGCAR format file from VASP. You could try if it provide info that you need.
http://theory.cm.utexas.edu/henkelman/code/bader/
Hi Dr. Pidatella,
It seems that determining the center is still difficult with both the tools you attached.... Anyway, thank you again for these kind help!!
Best wishes,
Zhu
Hi Zhu,
I was thinking about the possibility to get our goal, i.e. the center of charge distribution by using the code that I attached in the previous message. The output file ACF.dat should contain the coordinates of each atom and the charge associated with it according to Bader partitioning. There's not a row with the center of charge, but you can calculate it for every Cartesian direction (x,y,z) simply through this expression:
[atomic associated charge value x atomic coordinate (x,y,z)] / [sum over all charge values],
i.e., a simple weighted sum, like that for getting center of mass.
Let me know if it works.
Best,
A.
Hi Dr. Pidatella,
I think I've misunderstand the meaning of ACF.dat. According to your comments, I can write a script to deal with the data like calculating the mass center. Thank you very much!!
PS: Do you think the position of the dipole center will influence the final results like dos, charge distribution, etc? Since the dipole center is only used for correction, its electric field strength is set automatically by VASP which is related to the external electric field and position of the dipole.
Best wishes,
Zhu
Hi Zhu,
following the definition of dipole correction,
DIPOL = center of cell (in direct, fractional coordinates) --> R{center}
As you know, this tag determines the center of the net charge distribution. The dipole is defined as:
$\int(r-R_{center})\rho_{ions+valence}rd^{3}r$
In the integral it represents a constant that you can take out of the integral. It's just a multiplier. If the flag is not set, VASP determines, where the charge density averaged over one plane drops to a minimum and calculates the center of the charge distribution by adding half of the lattice vector perpendicular to the plane where the charge density has a minimum (this is a rather reliable approach for orthorhombic cells).
In order to answer to your question, probably there shouldn't be a so big difference in the final results due to the fact that it's possible to guess that the default DIPOL setting it's not too far from finding the real center of charge. But anyway, it's worth a try!
Please, keep me posted on your results.
Best,
A.
Hi Dr. Pidatella,
Thank you for your post and I also think the DIPOL setting should not make the result varies much. Surely, the test will cost much time (lol), but once I get the results, I will post them here.
Best wishes,
Zhu
Hi There,
Could you tell me how can we apply electric field in VASP?
Thank you.
Hi There,
I think I have found the answer. The basic principle is to keep the dipole at the middle sites of vacuum region.
Thus, for a slab located in the middle of cell, set DIPOL = 0.5 0.5 0.5 is applicable.
Cheers
This makes it sound like it really should be the "center of mass" which is essentially the center of the slab. Is this correct?
Hi.
In VASP INCAR i have to specify the EFIELD_PEAD tag in eV/ang unit for homogeneous electric field. But in the papers I found the they have used V/nm unit. I would be grateful If any one could guide me in this matter
@Atasi:
Please see how energy of an electron is calculated in an external electric field.
and relate that with definition of electric field.
The answer is that the two units mentioned, differ only by a multiplicative constant.
@Vaibhav Kaware : of course both are connected by a multiplicative constant and my concern is that .. it is of the order of 10^19 . is it really so, that i have to specify such a big no as input .. or some thing else.. like it is taking the field value per electron or in some other way?
Well, since they have said so, I guess you have to. Besides, the number may be big, but notation is small e.g. ~1.9D-9 .
If you are worried about the convergence and crashing etc., and if it crashes at all, call it the first step and try to resolve afterwards.
From what I read, its just eV/Ang. No mention of per particle or cell or anything else.
Best.
# External EFIELD in VASP
EFIELD = [real no.] # Units of eV/A, one no only
IDIPOL=1-3 #Applied Electric field direction 1-3(one no.)
LDIPOL=.TRUE. # Avoid dipolar interation btwn periodic images
With a single point energy (static) calculation INCAR set the above flags.
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