[Pw_forum] Fermi energy reference point shift from 0 to -2
Filipe Camargo Dalmatti Alves Lima
flima at if.usp.br
Fri Apr 5 19:54:35 CEST 2013
Hi,
It is normal, the Fermi energy shows the energy of highest occupied
electron and it does not necessary is exactly on zero.
You can shift All energies by an additive constant without no problem. For
one single system it is ok to shift the energies by an additive constant.
(Like add +2.0508 to move the Fermi level to zero) and it is not changing
the physics of the result, just the way you are presenting.
However, BE CAREFUL! It dos not mean that you can align two different
systems using this method.
>From the QE website:
*
*
*“The value of the Fermi energy (as well as of any energy, for that matter)
depends upon the reference level. What you are referring to is probably the
“Fermi energy referred to the vacuum level” (i.e. the work function). In
order to obtain that, you need to know what the vacuum level is, which
cannot be said from a bulk calculation only”>* (Stefano Baroni, Sept. 2008).
http://www.quantum-espresso.org/?page_id=364
Best regards,
Filipe Lima - PhD Student - University of São Paulo, Brazil.
On 04/04/2013, at 22:30, Shamsu Abubakar <say457 at yahoo.co.uk> wrote:
Thank you Mr. Robert for your kind response.
I will include the verbosity='high' and repeat the calculations. However,
you made mention that, when i plotted the DOS there is need to shift my
Fermi level to zero; that i did not understand very well. My question is,
How to shift my Fermi to zero?. Secondly, I always see my Fermi energy at
the end of my scf calculation.'' the Fermi energy is -2.0508'', before
total energy line. Does it mean -2.0508 is my actual Fermi energy? because
the zero point in the DOS graph is just a reference point. Lastly, for my
plotting software, i used Qtiplot. Usually, after my ./dos.x input and
output, i take my 'dos' directly to plot my DOS. Therefore, I don't know
if you or someone can help with some guide about it.
Once again thank you and Best regards.
Shamsu Abubakar
Postgraduate student
Department of Physics
University Putra Malaysia
------------------------------
*From:* Robert Hembree <hembreerofphysics at gmail.com>
*To:* 'Shamsu Abubakar' <say457 at yahoo.co.uk>; 'PWSCF Forum' <
pw_forum at pwscf.org>
*Sent:* Friday, 5 April 2013, 1:17
*Subject:* RE: [Pw_forum] Fermi energy reference point shift from 0 to -2
Dear Shamsu
Your input file and parameters are fine. When you plot the DOS you need to
shift your Fermi level to zero if you want the gap to show up at zero.
Add the line
verbosity = ‘high’
to the control part of your input file and run pw.x again. Then you can
grep your output file
less scf.out | grep ‘Fermi energy’
You should get a line that looks like this:
the Fermi energy is -1.6709 ev
When you plot your DOS, for instance in gnuplot you would now plot
plot ‘Graph01.dos’ ($1-(-1.6709)):2 w l
and you should see your plot with the bandgap and dispersion at zero.
When you do these calculations the Fermi level will rarely show up at
exactly zero.
Robert
*From:* pw_forum-bounces at pwscf.org
[mailto:pw_forum-bounces at pwscf.org<pw_forum-bounces at pwscf.org>]
*On Behalf Of *Shamsu Abubakar
*Sent:* Wednesday, April 03, 2013 8:23 PM
*To:* pw_forum at pwscf.org
*Subject:* [Pw_forum] Fermi energy reference point shift from 0 to -2
Dear Q.E Users.
I have a system of 4x4 graphene supercell, comprising 32 carbon atoms. I
sucessfully run all the calculations i.e scf,nscf and other postprocessing
such as DOS and PDOS. But the main problem i'm facing currently is on the
density of state. Usually, i consider zero to be my fermi reference energy
point in my ./dos.x input file, forexample,from -10 and 10 as my Emin and
Emax respectivily. For pristine graphene,i expect to always see the
dispersion at the zero vicinity, showing a semi metallic behavior, but
surprisingly, its always moving away from zero to around -2, in the valence
band side which i knew is unsual for pristine graphene. However, when i
combine graphene with GaAs nanostructure, most of the interesting
properties interms of orbitals contributions (PDOS), also resides on
similar point -2. That is some of the contributions that are supposed to be
seen at the vicinity of the fermi level, now moved to another point -2.
Also, i consider changing the smearing from marzari-vanderbilt to Fermi-
Dirac, but similar properties were observed. I decided to consider only
1x1, and 2x2 graphene supercell alone, still similar properties i observed
also. In fact, i even consider changing my pseudopotential from pw91 to
pbe.rrkjus still no progress. My problem now, i don't know what really
causes these shift from 0 to -2.
I attached one of my graphene scf input file for your verification and
advice.
&CONTROL
title = 'Graphene monolayer01',
prefix = 'Graph01',
calculation = 'scf' ,
restart_mode = 'from_scratch' ,
outdir = '/home//???/espresso-4.1.2/bin/' ,
pseudo_dir = '/home//???/???/espresso-4.1.2/upf_files/' ,
tstress = .true. ,
tprnfor = .true. ,
/
&SYSTEM
ibrav = 4,
celldm(1) = 18.59490373,
celldm(3) = 1.80,
nat = 32,
ntyp = 1,
ecutwfc = 34.D0 ,
tot_charge = 0.000000,
occupations = 'smearing' ,
degauss = 0.02 ,
smearing = 'fermi-dirac' ,
/
&ELECTRONS
conv_thr = 1.D-8 ,
mixing_beta = 0.7D0 ,
/
ATOMIC_SPECIES
C 12.00000 C.pw91-van_ak.UPF
ATOMIC_POSITIONS (angstrom)
C 1.229999657 0.709326753 1.999999753
C 2.459999165 1.419486000 1.999999874
C 3.689999208 0.709327391 1.999999822
C 4.920000000 1.419486496 1.999999944
C 4.920000000 2.839880146 2.000000125
C 6.149999990 3.549976521 1.999999907
C 3.690000010 3.549976521 1.999999907
C 2.459999764 2.839879152 2.000000067
C 6.150000792 0.709327391 1.999999822
C 7.380000835 1.419486000 1.999999874
C 7.380000236 2.839879152 2.000000067
C 8.610000005 3.549975943 1.999999891
C 8.609999654 4.970023341 2.000000208
C 9.840000000 5.680120078 2.000000200
C 7.379999382 5.680120770 2.000000152
C 6.149999800 4.970024086 2.000000150
C 4.920000000 5.680121474 2.000000132
C 3.690000200 4.970024086 2.000000150
C 8.610000343 0.709326753 1.999999753
C 9.840000000 1.419485253 1.999999874
C 9.840000000 2.839878353 2.000000070
C 11.069999995 3.549975943 1.999999891
C 11.070000346 4.970023341 2.000000208
C 12.300000618 5.680120770 2.000000152
C 12.300001137 7.100514050 2.000000166
C 13.530000464 7.810673002 1.999999744
C 11.070000830 7.810672948 1.999999928
C 9.840000000 7.100514025 2.000000287
C 8.609999170 7.810672948 1.999999928
C 7.379998863 7.100514050 2.000000166
C 6.149999536 7.810673002 1.999999744
C 4.920000000 7.100514261 2.000000041
K_POINTS automatic
6 6 1 0 0 0
Thank you for your kind response.
Shamsu Abubakar
Postgraduate student
Department of Physics
University Putra Malaysia
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