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

[Stefano Baroni]

Lorenzo: I beg to differ with you. It would be absurd indeed to treat an electron as a classical particle. This would break not only several “implicit assumptions” made in the code, but, I am afraid, an entire century of honoured service of Quantum Mechanics. (the reason why “pseudizing” the entire electron charge distribution of an inert gas dos not work is technical, not conceptual, IMHO. There reason why electrons behave quantum mechanically is deeper, and decidedly conceptual). SB

On 29 Aug 2024, at 15:46, Alex Kirkpatrick <canilivehere at hotmail.co.uk> wrote:

That makes a lot of sense. I really appreciate you taking the time to explain all of that.

All the best,
Alex

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________________________________
From: Lorenzo Paulatto <lorenzo.paulatto at cnrs.fr<mailto:lorenzo.paulatto at cnrs.fr>>
Sent: 29 August 2024 13:37
To: Alex Kirkpatrick <canilivehere at hotmail.co.uk<mailto:canilivehere at hotmail.co.uk>>; 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?


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,

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________________________________
From: Lorenzo Paulatto <lorenzo.paulatto at cnrs.fr><mailto:lorenzo.paulatto at cnrs.fr>
Sent: 29 August 2024 11:27
To: Alex Kirkpatrick <canilivehere at hotmail.co.uk><mailto:canilivehere at hotmail.co.uk>; 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?


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

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________________________________
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

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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
--
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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