[SIESTA-L] Graphene and symmetry, new user question

Fri, 12 Mar 2010 18:08:57 -0800 

Hi all!
I am a new user of Siesta and I'm involved in graphene band structure
calculations. I can not obtain the correct band structure for a graphene
nanoribon (N=3m+2, armchair passivated by H-atoms). Trying to do that I
was overwhelmed  with the fact that the system absolutely symmetrical in
Y-axis (finite size, 5 honeycomb rings) has non-zero dipole moment along
Y-axix which depends sharply on the Meshcutoff value.
Could anyone explain me where it comes from? What are the possible sources
of this "effect" and how it may affect the band structure?

The replica of my .fdf and .out files are attached below.

Thanks for any reasonable help.

>From input-file

NumberOfAtoms       26
NumberOfSpecies     2

%block ChemicalSpeciesLabel
 1  6  C      # Species index, atomic number, species label
 2  1  H
%endblock ChemicalSpeciesLabel
Meshcutoff     100.00 Ry
LongOutput T
WriteDenchar T
AtomicCoordinatesFormat  Ang
%block AtomicCoordinatesAndAtomicSpecies
  0.00         7.37853644     0.000   1
  1.42         7.37853644     0.000   1
 -0.71         6.148780367    0.000   1
  2.13         6.148780367    0.000   1
  0.00         4.919024294    0.000   1
  1.42         4.919024294    0.000   1
 -0.71         3.68926822     0.000   1
  2.13         3.68926822     0.000   1
  0.00         2.459512147    0.000   1
  1.42         2.459512147    0.000   1
 -0.71         1.229756073    0.000   1
  2.13         1.229756073    0.000   1
  0.00         0.00           0.000   1
  1.42         0.00           0.000   1
 -0.71        -1.229756073    0.000   1
  2.13        -1.229756073    0.000   1
  0.00        -2.459512147    0.000   1
  1.42        -2.459512147    0.000   1
 -0.71        -3.68926822     0.000   1
  2.13        -3.68926822     0.000   1
  0.00        -4.919024294    0.000   1
  1.42        -4.919024294    0.000   1
  0.00         5.889024294    0.000   2
  0.00        -5.889024294    0.000   2
  1.42         5.889024294    0.000   2
  1.42        -5.889024294    0.000   2
%endblock AtomicCoordinatesAndAtomicSpecies

%block PAO.BasisSizes
   C     DZP
   H     DZP
%endblock PAO.BasisSizes

LatticeConstant  2.459512147  Ang
%block LatticeVectors
  1.732050808  0.00    0.0
  0.0          80.0    0.0
  0.0          0.00    80.0

%endblock LatticeVectors

%block BandLines
1   -1.0   0.0  0.0
20   0.0   0.0  0.0
20   1.0   0.0  0.0
%endblock BandLines


>From out-file

siesta: Electric dipole (a.u.)  =    0.000000   17.412033    0.000000
siesta: Electric dipole (Debye) =    0.000000   44.257004    0.000000


Artem Baskin,
PhD student,
University of Illinois at Chicago

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