# [QE-users] dipfield

Tue Nov 13 14:50:03 CET 2018

```Thank you Thomas, I will try both flag one by one and see what change do
each of them brings separately.

On Tue, Nov 13, 2018 at 3:52 PM Thomas Brumme <thomas.brumme at uni-leipzig.de>
wrote:

>
>
> No! It's either dipfield = .true. or assume_isolated ='2d'. If you want to
> simulate "a real" 2D system using
> the Coulomb cutoff you don't need to correct due to the systems dipole.
> And if you correct with the artificial
> dipole, cutting of the Coulomb will result in something bad/weird...
> Actually, I don't know if this will work at all.
>
> So, either dipfield = .true. or assume_isolated ='2d'
>
> Thomas
>
> On 11/13/18 11:45, Asad Mahmood wrote:
>
> Thanks Thomas for reply. So I need to include both flags(i.e dipfielf =
> .true and assume_isolated ='2d' ) in relax or structure optimization
> calcations ?
>
> On Tue, Nov 13, 2018, 3:11 PM Thomas Brumme <thomas.brumme at uni-leipzig.de
> wrote:
>
>>
>> What is the benefit of using dipfield=.true.? Well, the benefit is that
>> you get the correct electronic structure,
>> energetics, and so on. As you want to simulate a 2D system with 3D
>> boundary conditions this is the only
>> correct way if you have an asymmetric system (in the z-direction,
>> assuming the 2D system is in xy). Well,
>> not the only correct way. Recently a Coulomb cutoff technique was
>> implemented in QE, check input flag
>> assume_isolated='2D'
>>
>> Regards
>>
>> Thomas
>>
>> P.S.: If you have a dipole in your system you'll see that properties
>> (especially the total energy) will change
>> with increasing vacuum region. If you increase it, you'll minimize the
>> eigenfunction overlap between the
>> repeated layers but the Coulomb interaction will never be zero and thus
>> you can get wrong results - and
>> this will be worse for larger dipole, e.g., the flourinated graphene.
>>
>> On 11/13/18 08:38, Asad Mahmood wrote:
>>
>> Hi,
>>
>> I have a hexagonal graphene(or any other) monolayer and I hydrogenate(or
>> flourinate, etc.) on the top surface only (single sided). Is there need to
>> use flag dipfield = .true ? What is benefit or disadvantage of using this
>> flag for optimization of L.C or bond length?
>> I am using 20 angstrom vacuum between two layers to avoid mutual
>> influence.
>>
>>
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>>
>>
>> --
>> Dr. rer. nat. Thomas Brumme
>> Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry
>> Leipzig University
>> Phillipp-Rosenthal-Strasse 31
>> 04103 Leipzig
>>
>> Tel:  +49 (0)341 97 36456
>>
>> email: thomas.brumme at uni-leipzig.de
>>
>>
> --
> Dr. rer. nat. Thomas Brumme
> Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry
> Leipzig University
> Phillipp-Rosenthal-Strasse 31
> 04103 Leipzig
>
> Tel:  +49 (0)341 97 36456
>
> email: thomas.brumme at uni-leipzig.de
>
>
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