[QE-users] PbTiO3 slab in vacuum with assume_isolated='2D'

[Lucian D. Filip]

Hi Lorenzo,

Thank you for your reply!

The slab is in the center of the cell, not centered around z=0 like it 
is the requirement for the assume_isolated=esm case. Should I center it 
around z=0 if the step function is added at half the cell z-length? In 
my case that position is in the middle of the slab, if I understand 
correctly.

Related to this, I have another question about the vaccum surrounding 
the slab. I am attaching another picture where the same PTO slab (in the 
same polarization state) is first surrounded by 20 Angstrom of vacuum on 
each side and then by 10 Angstrom on each side. In the second case, the 
Hartree potential appears to also be "corrected" inside the slab. This 
is how I would expect a fully compensated ferroelectric to behave and 
this is what it is obtained if compensation charges are placed at the 
ends of the slab in the same manner as presented by Javier Junquera 
here: 
https://personales.unican.es/junqueraj/JavierJunquera_files/Metodos/Constrained-D/Exercise-constrained-D.pdf 
(the method is exemplified for Siesta).

My question is: when I am choosing the vacuum region for the 
assume_isolated='2D' flag, do I aim for the result in the 20 angstrom 
case? What is the best practice here?


Thank you for taking the time,

Best regards,


Lucian

On 25-Jun-24 8:14 AM, Lorenzo Paulatto wrote:
>
> Hello Lucian,
>
> is the slab in the center of the cell, or around z=0 ?
>
> Anyway, the option assume_isolated='2D' modifies the Hartree 
> interaction, adding a cut-off smooth step function at half the cell 
> z-length, i.e. 1/r → step(z)/r. If a long-range 1/r^n propagates 
> outward from your slab, at half the cell distance you will see the 
> step function. As long as this happens in a region where there is 
> vacuum, it is not a problem.
>
> kind regards
>
> On 24/06/2024 13:09, Lucian D. Filip wrote:
>>
>> Hello,
>>
>> I am looking at a PbTiO3 slab in vacuum. The slab is in a polarized 
>> state and I am trying to obtain the Hartree potential with the dipole 
>> correction applied. I have seen that the assumed_isolated = '2D' 
>> option should do just that. However, I do not understand the result I 
>> am getting. In the attached picture I have plotted the Hartree 
>> potential averaged along the z axis, for the polarized state and the 
>> centro-simmetric state with polarization zero.
>>
>> The slab is 5 PTO unit cells thick and there are 20 Angstroms of 
>> vacuum on each side. The vacuum region is well above the requirements 
>> of assume_isolated ='2D'. Can anyone explain two things:
>>
>> 1) why are there two regions in vacuum looking like a staircase.
>>
>> 2) why is there an electric field still present?
>>
>>
>> The weird artefacts at the edges of the supercell are due to the 
>> averaging procedure (i am still working on it, not sure where it is 
>> coming from).
>>
>>
>> Best regards,
>>
>> Lucian
>>
>>
>> -- 
>> Dr. Lucian Dragos Filip
>> National Institute of Materials Physics
>> Atomistilor str. 405A, PO Box MG. 7
>> Magurele, 077125
>> Bucharest, Romania
>> E-mail:lucian.filip at infim.ro
>> Website:https://lucianfilip.wordpress.com/
>>
>> _______________________________________________
>> The Quantum ESPRESSO community stands by the Ukrainian
>> people and expresses its concerns about the devastating
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>> country and on the free and peaceful scientific, cultural,
>> and economic cooperation amongst peoples
>> _______________________________________________
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> -- 
> Dr. Lorenzo Paulatto
> IdR @ IMPMC - CNRS UMR 7590 & Sorbonne Université
> phone: +33 (0)1 442 79822 / skype: paulatz
> http://www.impmc.upmc.fr/~paulatto/ - https://anharmonic.github.io/
> 23-24/423 B115, 4 place Jussieu 75252 Paris CX 05
>
> _______________________________________________
> The Quantum ESPRESSO community stands by the Ukrainian
> people and expresses its concerns about the devastating
> effects that the Russian military offensive has on their
> country and on the free and peaceful scientific, cultural,
> and economic cooperation amongst peoples
> _______________________________________________
> Quantum ESPRESSO is supported by MaX (www.max-centre.eu)
> users mailing listusers at lists.quantum-espresso.org
> https://lists.quantum-espresso.org/mailman/listinfo/users

-- 
Dr. Lucian Dragos Filip
National Institute of Materials Physics
Atomistilor str. 405A, PO Box MG. 7
Magurele, 077125
Bucharest, Romania
E-mail:lucian.filip at infim.ro
Website:https://lucianfilip.wordpress.com/
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