[QE-users] dipole correction - "saggy" electrostatic potential?

[Dr. Thomas Brumme]

Dear Chris,

adding to my last reply: in the end, it all, of course, depends on what
you want to simulate. If you're aiming for a charged species on a surface
then assume_isolated='2D' might be wrong if you don't increase the slab
(mimicking the surface of a bulk material) such that the electric field
at the bottom does not influence the top. It might be worth to also look
at my implementation of a gate setup (gate=.true.) or to wait till the
author of assume_isolated='2D', Thibault Sohier, releases his implementation
including the gate setup. Gate? Maybe check this paper to see why this
could be useful:

https://www.sciencedirect.com/science/article/pii/S0169433218315022

As I said, it all depends on the situation you want to simulate. But a
flat potential with the same absolute value on both sides of the system
is rarely correct for a charged system.

Regards

Thomas

Zitat von Christoph Wolf <wolf.christoph at qns.science>:

> Dear Thomas,
>
> I played a bit with "assume_isolated='2D'" but I do not think that this can
> correct the electrostatic potential of charged sytems (in the sense that
> the potential becomes "flat") unless I am interpreting the output
> (attached) wrong.
>
> One way that gives me a flat vacuum potential is to use the M-P scheme but
> that only works for cubic systems. After reading about the implementation
> in VASP a bit I also think that is what they recommend.
>
> Best,
> Chris
>
> On Thu, Jun 28, 2018 at 7:04 PM, Dr. Thomas Brumme <
> thomas.brumme at uni-leipzig.de> wrote:
>
>> Dear Chris,
>>
>> The potential shows the typical quadratic dependence on z since you're
>> calculating a charged system - there is a homogeneous background charge
>> since the 3D pbc system is assumed to be neutral. This has nothing to do
>> with the dipole correction. Depending on what you want to do next it might
>> be useful to set the flag assume_isolated='2D'
>>
>> Regards
>>
>> Thomas
>>
>> Zitat von Christoph Wolf <wolf.christoph at qns.science>:
>>
>>
>> Dear all,
>>>
>>> I am still observing something strange in my slab + dipole correction
>>> calculation that I do not fully understand.
>>>
>>> When using dipfield+tefield (eopreg and emaxpos well within the vacuum
>>> region) I encounter a "saggy" electrostatic potential (plot_num=11)
>>> despite
>>> the sawtooth efield potential (plot_num=12) looking as usual. Maybe
>>> someone
>>> can give it a look and confirm if this is due to the excess charge in the
>>> system (this does not happen when running the same system in VASP)?
>>>
>>> I attach input and the plot of the potential for 2 and 4 layers of vacuum
>>> (more vacuum does seem to improve the situation).
>>>
>>> Thank you in advance for your time and assistance!
>>>
>>> Chris
>>>
>>> PS: I am using a QE version with dipole bug fix:
>>>
>>>      Program PWSCF v.6.2.2 starts on 25Jun2018 at 10:39:24
>>>
>>> --
>>> Postdoctoral Researcher
>>> Center for Quantum Nanoscience, Institute for Basic Science
>>> Ewha Womans University, Seoul, South Korea
>>>
>>
>> --
>> 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
>>
>>
>
>
> --
> Postdoctoral Researcher
> Center for Quantum Nanoscience, Institute for Basic Science
> Ewha Womans University, Seoul, South Korea