Different Electric Polarization

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Different Electric Polarization

Date: 2023/08/08 17:00
Name: Ella: Dear Developers,
I am performing NEB calculation and also berry phase to obtain ferroelectric polarization in my system. However, I keep getting a higher polarization value on the paraelectric state in comparison to the polarization of ferroelectric state. I also found that the atoms are a bit shifted during the NEB calculation although I have made it fixed.
And also, if I compare the ferroelectric polarization value by using the position I obtain from the NEB (neb_0.xyz) with the position I obtain from the geometry optimization, the results are different one to another.

I wish you could help me with this.

Regards,
Ella

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Re: Different Electric Polarization ( No.1 )

Date: 2023/08/08 23:32
Name: Naoya Yamaguchi

Hi,

>However, I keep getting a higher polarization value on the paraelectric state in comparison to the polarization of ferroelectric state.

You should find the net value of electric polarization, and the absolute value of electric polarization is meaningless in solids.

Regards,
Naoya Yamaguchi

Re: Different Electric Polarization ( No.2 )

Date: 2023/09/12 10:11
Name: Ella

I am trying to obtain the net value by taking the difference of the polarization I obtained in ferroelectric state and in para-electric state. I am calculating the polarization under the effect of strain. based on the polarization trend of several percents, the net value polarization get increased following the addition of strain. However, I keep getting a strange value in certain percent addition of strain because the ferroelectric state polarization value will suddenly drop.

Re: Different Electric Polarization ( No.3 )

Date: 2023/09/12 17:09
Name: Naoya Yamaguchi

Dear Ella,

If the phase becomes metallic, electric polarization cannot be defined.
You also need to consider the arbitrariness of 2*pi*e/(|G_i|V) in the G_i-component of electric polarization.
Note: the notation of the variables follows https://www.openmx-square.org/tech_notes/tech11-1_1.pdf .

Regards,
Naoya Yamaguchi

Re: Different Electric Polarization ( No.4 )

Date: 2023/09/13 12:51
Name: Ella

Dear Yamaguchi-Sensei,
My system does not become metallic and I already perceived that I have to consider the arbitrariness of 2*pi*e/(|G_i|V) in the G_i-component of electric polarization. Based on the reference, If I am not mistaken, I shall locate where the berry phase happens and then find the quantum phase. However, I don't have any idea about how to solve it related to my experimental data. Does it mean that I have to add something in my polB code?

Regards,
Ella

Re: Different Electric Polarization ( No.5 )

Date: 2023/09/13 21:15
Name: Naoya Yamaguchi

Dear Ella,

Since no specific information about your system is provided, in general terms, it is important to monitor the continuous change of the Berry phase. Furthermore, the convergence of k-point sampling in `polB` is relatively slow, so you should ensure the k-point convergence rigorously for quantitative discussions.

Regards,
Naoya Yamaguchi

Re: Different Electric Polarization ( No.6 )

Date: 2023/09/19 13:32
Name: Ella

Dear Yamaguchi-Sensei,
Thank you for your kind response. I could confirm that I obtained the convergence of berry phase with a relevant k-point for my system. However, I the result yielded shows me that the electron contribution in ferroelectric phase is negative in every direction. Therefore, the total polarization is reduced. Below I put the output as an example.

Background Core Electron Total

Px -0.00000000 183.38514388 -0.00000000 183.38514388
Py -0.00000000 459.54694983 -13.70193343 445.84501640
Pz -0.00000000 377.82391217 -0.00000000 377.82391217

I believe there is something about this which is related to the berry phase. I wish you could tell me what I am unaware of.

Regards,
Ella

Re: Different Electric Polarization ( No.7 )

Date: 2023/09/19 20:25
Name: Naoya Yamaguchi

Dear Ella,

The signs of the `Total` values in the output are not acceptable as is because the `Total` values are just the sum of the `Core` and `Electron` values, and you should consider the arbitrariness of the Berry phases (including the `Core` part) carefully.

Regards,
Naoya Yamaguchi

Re: Different Electric Polarization ( No.8 )

Date: 2023/10/13 17:56
Name: Ella

Dear Yamaguchi-Sensei,
Could you tell me how to consider the arbitrariness of the Berry phases in openmx? I am unsure that my comprehension is right, because this far I only displayed the hidden phase by activating the phase disappeared by modulo 2PI and sum it up with the core and electronic contribution. Still my problem is not solved yet.

I am looking forward to your response at your earliest convenience.

Regards,
Ella

Re: Different Electric Polarization ( No.9 )

Date: 2023/10/30 16:20
Name: Ella

Dear Yamaguchi-Sensei,
I have found out that my problem arises due to the arbitrariness of 2*pi*e/(|G_i|V) and it is resolved. However, I am unable to find the reasonable answer why the arbitrariness only happen in certain percentage of the additional strain.

Regards,
Ella

Re: Different Electric Polarization ( No.10 )

Date: 2023/10/30 21:21
Name: Naoya Yamaguchi

Dear Ella,

I'm glad to hear that you resolved the issue. I think that when the arbitrariness changes is not simple to find. I can't give comments without checking the contents of files in the case.

Regards,
Naoya Yamaguchi

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