[QE-users] Pseudopotential to approximate a free electron?

[Lorenzo Paulatto]

On 29/08/2024 14:25, Alex Kirkpatrick wrote:
> You've given me an interesting direction to follow, thanks for the info!
>
> Just for curiosity's sake, is the problem with the idea of 'adding a 
> free electron pseudo' because it means a very different thing in QE 
> code than 'adding a particle to the simulation that acts like an 
> electron'?


A pseudopotential is a modelisation of the atom nuclei + the core 
electrons which are assumed to be rigid, and their effect on the valence 
atoms. It is inherently a rigid object. You choose where to put the 
pseudopotentials in the unit cell, then the code adds the necessary 
amount of electrons to neutralize them, these electrons are free to 
delocalize and float around in order to minimize the total energy.

It is not totally completely absurd to conceive a pseudopotential like a 
point-charge has a negative charge, i.e. a potential of form  -q/r, then 
assign to it the electron mass and compute the force acting on it, but 
it would probably break a lot of implicit assumptions that have been 
made in the code.

For example, I once tried to do a pseudopotential for a noble gas with 
zero electrons in valence, and it totally did not work properly, what 
you propose is even more extreme.


hth


>
> Kind regards,
>
> Sent from Outlook <http://aka.ms/weboutlook>
>
>
> ------------------------------------------------------------------------
> *From:* Lorenzo Paulatto <lorenzo.paulatto at cnrs.fr>
> *Sent:* 29 August 2024 11:27
> *To:* Alex Kirkpatrick <canilivehere at hotmail.co.uk>; Quantum ESPRESSO 
> users Forum <users at lists.quantum-espresso.org>
> *Subject:* Re: [QE-users] Pseudopotential to approximate a free electron?
>
>
> On 29/08/2024 12:36, Alex Kirkpatrick wrote:
>
>     Thanks for the reply Lorenzo.
>
>     I'm very new to all this but my thought was that, to have some
>     control over the distance between the 'electron' and the surface,
>     I would need to include it as a discrete entity.
>
>
> Hello, QE is a code based on plane waves, which means that you cannot 
> choose where an electron goes. But I don't think you need to use an 
> explicit electron for your task. Thank to the power of numerical 
> simulation, you can plot/measure directly the magnitude of the 
> Kohn-Sham potential, (which is identical to the Hartree electrostatic 
> potential when you go in the vacuum).
>
> The procedure is similar to the one explained in the workfunction 
> example (PP/examples/WorkFct_example) which shows how to compute the 
> average value of the electrostatic potential far away in the vacuum to 
> estimate the energy required to pull an electron from the surface out 
> to infinity.
>
> Of course the procedure is not entirely self-consistent (i.e. pulling 
> out the electron would change the potential) but for a metal it is 
> very close, and if you really need to you can remove some amount of 
> charge fro mthe surface using a positive value for tot_charge
>
> hth
>
>
>     I would then be able to vary the distance and see how things
>     change. Is this the wrong way to go about it?
>
>     Kind regards
>     Alex
>
>     Sent from Outlook <http://aka.ms/weboutlook>
>
>
>     ------------------------------------------------------------------------
>     *From:* users <users-bounces at lists.quantum-espresso.org>
>     <mailto:users-bounces at lists.quantum-espresso.org> on behalf of
>     Lorenzo Paulatto <lorenzo.paulatto at cnrs.fr>
>     <mailto:lorenzo.paulatto at cnrs.fr>
>     *Sent:* 29 August 2024 10:15
>     *To:* Quantum ESPRESSO users Forum
>     <users at lists.quantum-espresso.org>
>     <mailto:users at lists.quantum-espresso.org>
>     *Subject:* Re: [QE-users] Pseudopotential to approximate a free
>     electron?
>     You could, probably, but why not just use tot_charge=-1
>
>     Kind regards
>
>
>     On August 29, 2024 10:03:30 AM GMT+02:00, Alex Kirkpatrick
>     <canilivehere at hotmail.co.uk>
>     <mailto:canilivehere at hotmail.co.uk> wrote:
>
>         Dear all,
>
>         Is it possible to create a pseudopotential file, or modify an
>         existing one like hydrogen, to approximate placing a free
>         electron in the simulation cell?
>         The end goal being to model its interaction with material
>         surfaces in vacuum.
>
>         Best wishes,
>         Alex
>
>         Sent from Outlook <http://aka.ms/weboutlook>
>
> --
> Dr. Lorenzo Paulatto
> IdR @ IMPMC - CNRS UMR 7590 & Sorbonne Université
> phone: +33 (0)1 442 79822 / skype: paulatz
> http://www.impmc.upmc.fr/~paulatto/ 
> <http://www.impmc.upmc.fr/~paulatto/> - https://anharmonic.github.io/ 
> <https://anharmonic.github.io/>
> 23-24/423 B115, 4 place Jussieu 75252 Paris CX 05
-- 
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
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