[Pw_forum] change in Fermi Energy with ecutoff, Vacuum and degauss

[Eduardo Ariel Menendez Proupin]

Deepak,
Periodic calculations have several components of the electrostatic energy
that are separately infinite, but added they cancel. Instead of dealing
with infinites, they are usually eliminated in plane wave codes setting to
0 the G=0 components of the Fourier transforms. I guess that Quantum
ESPRESSO  satisfies this rule. A component V(G=0)  is just the spatial
average of V(r), meaning in practice that  the electrostatic potential (the
Hartree part) is adjusted to zero. The amount of vacuum volume in the
supercell represents a weight for the spatial averages. If you change from
10 to 20 angstroms of vacumm width, you are doubling the weight of the
vacuum part in the average.

If the average were not adjusted to zero,  then I would expect that the
average tends softly to cero for infinite vacuum volume, and the Fermi
level converge to a constant value. As the average potential is
artificially shifted to zero, the Fermi level experiments the opposite
shift. For more interesting details, take a look at PRB 77, 115139 (2008).

Best regards


Eduardo Menendez Proupin
Departamento de Química Fisica Aplicada
Facultad de Ciencias
Universidad Autónoma de Madrid
28049 Madrid, Spain
Phone: +34 91 497 6706

---------- Mensaje reenviado ----------
From: Dipak Thakur <deepak.p.thakur at gmail.com>
To: pw_forum at pwscf.org
Date: Tue, 29 Nov 2011 22:05:00 +0530
Subject: [Pw_forum] change in Fermi Energy with ecutoff, Vacuum and degauss
Hello every one.

I am perplexed by change in Fermi energy with change in ecutoff, Vacuum and
degauss.

I have done some calculations using a set of ecutoff, Vacuum and degauss
and then repeated the same for other set of ecutoff, Vacuum and degauss.

I have obtained a value of -0.756 eV for Fermi energy in case where the
ecutoff was 60 Ry., Vacuum was 20 A and degauss was 0.02, whereas for the
second set with ecutoff of 20 Ry., Vacuum of 10 A and degauss of 0.002 the
Fermi energy was 3.0097 eV.

Can anyone help me which value of Fermi energy is correct and why?

Please help me in understanding the what wrong am I doing?

The calculations are related to graphene with Aluminium dopant (one atom of
Al and 31 carbon atoms).

Thanks and regards
Deepak Thakur
Research student
Swami Ramanand Teerth Marathwada University, Nanded.
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