[Pw_forum] Is the Graphite surface flat ?

Tue Jul 6 13:09:28 CEST 2010

Dear pwscf users

answer to the comments:
(many thanks for your kind attention and help but I could not solve the problem)

to G. Scluuzero: I have used the experimental C-C distance but then I have vc-relaxed the bulk cell and found the new positions. After vc-relax I have used 
these coordinates to "relax" the atoms in the cell with vacuum. Also the innermost layer is not graphite-like and is rippled.  

to D. C. Andera: this pseudopotential reproduces the experimental cell parameter of the bulk correctly and exactly. the predicted cell parameter in z-direction (c) by the other pseudopotentials is not so exact and sometimes, a difference about 1 angstrom compared to experimental value appears.

to G. Mattioli:
london=.true. closes the atoms in the different layers to each other in the relax calculation and did not solve the problem.

Now I am trying to relax a 5-layer sample and see the result. but it has not completed yet. really I do not know the surface of graphite is "ripple" in the nature (experimentally) or not? I appreciate your help.

> On 07/05/2010 08:19 PM, Masoud Nahali wrote:
> > Dear Quantum Espresso users
> >
> > I optimized the graphite bulk and the layers were completely flat. But 
> > when I try to "relax" the graphite surface (slab 2*2 with 3 monolayer) 
> > with 10 angstrom vacuum all the layers become extremely ripple!!! I 
> > can not find the problem. May it related to the number of layers? 
> > anyhow the number of layers is like the bulk. Your help would be a 
> > great gift.
G. Scluuzero:
> > __
> Are you using the experimental or the theoretical value for the in-plane 
> (graphene) C-C distance? I guess that the PBE value is larger than the 
> experimental one, so that if you start from the experimental C-C 
> distance in the graphene layer and let everything relax, then the atoms 
> would like to go towards the theoretical equilibrium distance and this 
> may cause the rippled structure.
> Secondly, I don't know if you can pretend that the innermost layer is 
> graphite bulk-like, having only three layers in total.

Dal Corso Andrea :

> Check the pseudopotential. This is a PP with a core hole for special
> purposes not for standard simulations.

 Giuseppe Mattioli :
> Dear Masoud
> apart from the pseudo problem issued by Andrea, you could try the keyword
> london=.true.
> It adds a Van der Waals contribution to total energy and it is likely to 
> improve the interlayer interactions in your system.
> Hope this helps
> Giuseppe 

input file:

&CONTROL

calculation  = "relax",
  pseudo_dir   = 
"/home/koa/soft/qe4.2/espresso-4.2/pseudo",
  outdir       = 
"/home/koa/tmp",
  etot_conv_thr= 1.0D-4,
  forc_conv_thr= 1.0D-3,

 nstep=150,
  /
&SYSTEM
  ibrav     = 4,
  a         = 
4.9318,
  b         = 4.9318,
  c         = 16.7029,

cosab     = -0.5,
  cosac     = 1.0,
  cosbc     = 1.0,

nat       = 24,
  ntyp      = 1,
  ecutwfc   = 40.D0,

ecutrho   = 400.D0,
  occupations = 'smearing'
  smearing ='mp', 

 degauss = 0.01,
  nspin = 2,
  starting_magnetization(1)= 1.0,

 /
&ELECTRONS
  conv_thr    = 1.D-6,
  mixing_beta = 0.7D0,

 diagonalization = "david",
/
&IONS
 ion_dynamics="bfgs"
/
ATOMIC_SPECIES
C 
 12.0107  C.star1s-pbe-rrkjus.UPF
ATOMIC_POSITIONS {angstrom}
C   
 0.000000    0.000000    0.000000
C    2.465900    0.000000    
0.000000
C    -1.232950    2.135532    0.000000
C    1.232950    
2.135532    0.000000
C    0.000000    1.423688    0.000000
C    
-1.232950    3.559220    0.000000
C    2.465900    1.423688    
0.000000
C    1.232950    3.559220    0.000000
C    0.000000    
0.000000    3.351450
C    2.465900    0.000000    3.351450
C    
-1.232950    2.135532    3.351450
C    1.232950    2.135532    
3.351450
C    1.232950    0.711844    3.351450
C    0.000000    
2.847376    3.351450
C    3.698850    0.711844    3.351450
C    
2.465900    2.847376    3.351450
C    0.000000    0.000000    
6.702900
C    2.465900    0.000000    6.702900
C    -1.232950    
2.135532    6.702900
C    1.232950    2.135532    6.702900
C    
0.000000    1.423688    6.702900
C    -1.232950    3.559220    
6.702900
C    2.465900    1.423688    6.702900
C    1.232950    
3.559220    6.702900
K_POINTS {automatic}
4 4 1 1 1 1

out
 put (optimized positions):

C        0.000000000  
-0.000000000  -0.686394528
C        2.465900000  -0.003714144  
-0.758610095
C       -1.229733457   2.137389072  -0.758610095
C       
 1.229733457   2.137389072  -0.758610095
C        0.000000000   
1.427474855  -0.213813075
C       -1.229670488   3.557326573  
-0.213813075
C        2.465900000   1.423688000  -0.285079086
C       
 1.229670488   3.557326573  -0.213813075
C       -0.000000000   
0.000000000   3.630366631
C        2.465900000  -0.014904710   
3.544792504
C       -1.220042143   2.142984355   3.544792504
C       
 1.220042143   2.142984355   3.544792504
C        1.220249305   
0.704511250   3.106073641
C        0.000000000   2.847376000   
3.022325711
C        3.711550695   0.704511250   3.106073641
C       
 2.465900000   2.862041499   3.106073641
C        0.000000000   
0.000000000   7.468882932
C        2.465900000   0.008344370   
7.466151214
C       -1.240176436   2.131359815   7.466151214
C       
 1.240176436   2.131359815   7.466151214
C        0.000000000   
1.414904155   6.963129179
C       -1.240557033   3.563611923   
6.963129179
C        2.465900000   1.423688000   6.961528237
C       
 1.240557033   3.563611923   6.963129179

Many Thanks
Masoud Nahali
Sharif
 University of Technology

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