[Pw_forum] Bilayer graphene subject to an electric field

[Eyvaz Isaev]

Hi,

A  couple of comments that might be useful.

1. As you use an US pseudopotential, reduce ecutwfc down to ~30Ry. I do not see 
any reason for such kind of high cutoff energy, unless you performed some tests 
and proved that 60Ry is better. Besides, for the same reason (US PsP), you 
should use more ecutrho (8-12 times of  ecutwfc). By default ecutrho=4*ecutwfc 
(why so, think about) and this works for norm-conserving PsPs. 


As a reference see  N.Mounet and N. Marzari, Phys. Rev. B 71, 205214 (2005), 
where they used ecutoff=40Ry for graphene and other C-based structures.

2. I have noticed that the job started to converge when I used smaller 
efield_cart(3)=0.05,  after 49  iteration  I had

     estimated scf accuracy    <       0.00007068 Ry

but then my notebook turned off due to a heating problem, I suggest. As I just 
tried to test the problem I used only 2x2x1 k-points set, and 
mixing_beta(1)=0.05. 

But when I used   efield_cart(3)=0.005 (with mixing coefficient  0.1) the job 
finished after 11 iterations. 



Hope this helps.

Bests,
Eyvaz. 
-------------------------------------------------------------------
Prof. Eyvaz Isaev, 
Department of Physics, Chemistry, and Biology (IFM), Linkoping University, 
Sweden 

Theoretical Physics Department, Moscow State Institute of Steel & Alloys, 
Russia, 

isaev at ifm.liu.se, eyvaz_isaev at yahoo.com




________________________________
From: Elie Moujaes <elie.moujaes at hotmail.co.uk>
To: pw_forum at pwscf.org
Sent: Mon, December 13, 2010 1:57:36 PM
Subject: [Pw_forum] Bilayer graphene subject to an electric field

  Dear all,
 
I am trying to get the band structure of bilayer graphene under the effect of an 
external electric field. I have done the scf calculations without the inclusion 
of an electric field , then again did teh scf calculation with an electric field 
included in the z-direction (of value 0.1 Ry). However the convergence is not 
achieved and it stopped after 100 iterations giving the following message:
 
 

     total cpu time spent up to now is  83435.11 secs

     total energy              =     -52.83248336 Ry
     Harris-Foulkes estimate   =     -45.80937450 Ry
     estimated scf accuracy    <       0.00524127 Ry

     End of self-consistent calculation

     convergence NOT achieved after 100 iterations: stopping

 
You will find below the inout file for the scf calculation when an electric 
field is applied:
 
     &control
    prefix='bielgraphene',
    calculation='scf',
    restart_mode='from_scratch',
    lelfield = .true.,
    nberrycyc= 1,
    pseudo_dir = '/espresso-4.2/pseudo/',
    outdir='/tmp/results_MOUJAES/'
 /
  &system    
    ibrav=  4, celldm(1) =4.608737, celldm(3)=5.330410, nat=4, ntyp= 1,
    ecutwfc = 60.D0,occupations='smearing',smearing='methfessel-paxton', 
degauss=0.01
/
 &electrons
    conv_thr=1.D-5,     
    mixing_mode='local-TF'
    mixing_beta=0.1D0
    efield_cart(1) = 0.000,
    efield_cart(2) = 0.000,
    efield_cart(3) = 0.10,
    
 /
ATOMIC_SPECIES
 C  12.0107  C.pz-rrkjus.UPF
ATOMIC_POSITIONS crystal
 C 0.000000  0.000000  0.000000  
 C 0.000000  0.000000  0.257692 
 C 0.333333  -0.33333  0.000000  
 C -0.333333  0.33333  0.257692  
 
K_POINTS automatic
   25 25 1  0 0 0  
 
 
Thanks
 
Elie Moujaes
University of Nottingham
NG7 2RD
UK 


      
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