[gpaw-users] questions about the metallic electrode in the poissonsolver

[Mikael Kuisma]

Hi!

You have surface charge density of 0.5 e / 6.31Å^2 =0.079 electrons / Å^2, which leads to

((0.079 * elementary charge) / (1 (Å^2))) / electric constant =
1.42951513 × 10^11 V / m

electric field. This is over 100 gigavolts per meter, so well beyond the typical physical electron field emission threshold. So it might be a real effect that the electron tries to fly away, but of course is trapped to this saw tooth like well between the boundary and the constant slope. The physical reason is that the fermi energy goes beyond the vacuum level, and the system stops accepting electrons.

Note that sometimes, spin polarization is required to relax charged systems.

Positive charges are easier, as you are not haunted with electron field emission. (but at some point, if you relax, with ion field emission).


BR,
Mikael


________________________________
From: gpaw-users <gpaw-users-bounces at listserv.fysik.dtu.dk> on behalf of Geng Sun via gpaw-users <gpaw-users at listserv.fysik.dtu.dk>
Sent: Thursday, August 15, 2024 11:10 AM
To: gpaw-users at listserv.fysik.dtu.dk <gpaw-users at listserv.fysik.dtu.dk>
Subject: [gpaw-users] questions about the metallic electrode in the poissonsolver

Dear GPAW users,
I would like to calculate the adsorption energies of small molecules on charged metallic slab.
I read that there is a metallic electrode method in GPAW. It seems to be useful to setup a charged slab with 2D periodicity.
I carried out a test calculation using Ru(0001) surface with one addtional electron, comparing the obtained electron density of the charged system and the neutral one. However, I found that the addtional electrron is located on the vacuum region at the top of the simulation box. I do not understand the results, I expected the electrons located around the Ru atoms.
I attached the script and the charge difference plot in the email. Is there any thing wrong in the calculation or is this a bug in the GPAW?
Thank you very much in advance.

Best
Geng



from gpaw import GPAW, restart
from ase.build import hcp0001
from gpaw.poisson import PoissonSolver
import numpy as np
from ase.units import Bohr
from ase.io import read, write


for charge in [0.0, -1.0]:
    slab=hcp0001('Ru',size=(1,1,4), vacuum=10.0)
    slab.set_pbc([True,True,False])
    if charge < -0.05:
        slab.calc = GPAW(
            mode='fd',
            xc='PBE', h=0.2,
            txt='charged.txt',
            charge=charge,
            kpts=(12, 12, 1),
            poissonsolver=PoissonSolver(metallic_electrodes='single'))
    else:
        slab.calc = GPAW(
            mode='fd',
            xc='PBE', h=0.2,
            txt='netural.txt',
            charge=charge,
            kpts=(12, 12, 1))
    slab.get_potential_energy()


    rho = slab.calc.get_all_electron_density(gridrefinement=4)
    write('q{:0.1f}.cube'.format(charge), slab, data=rho * Bohr**3)
    # slab.calc.write("optimized.gpw")


[cid:a6d5b0be-7af2-4af6-9899-3ad29b4a80ba]
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