[Pw_forum] Is the Graphite surface flat ?

Giuseppe Mattioli giuseppe.mattioli at mlib.ism.cnr.it
Tue Jul 6 12:22:00 CEST 2010


Dear Masoud

> 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.

I do not know if you catch the point... You are using a pseudopotential which 
contains a core hole into the C 1s shell. it CAN NOT be used to simulate in a 
proper way the graphite bulk! You will collect (actually, have collected...) 
weird results! You have luckily obtained a better interlayer distance because 
DFT generally fails when dealing with graphite, where the layers are not 
connected by covalent bonds. So, if you use a proper C pseudopotential, you 
achieve poor results for the c parameter in a vc-relax calculation.

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

Have you tryed to perform a vc-relax calculation of the graphite bulk by using 
a standard pseudopotential (us or nc, as you like) and by switching on the 
london=.true. keyword and, after that, to perform a relax calculation with 
the same pseudo and at the cell a and b parameters you found in the previous 
vc-run in order to optimize your tri-layer graphite slab?

hth

Giuseppe


On Tuesday 06 July 2010 13:09:28 Masoud Nahali wrote:
> 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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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
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