Artem,
I might be terribly wrong mistaken, but your geometry doesn't look like
it's symmetric. Look at the y-coordinates of your H-atoms. They are
about +5.8 and -5.8.
Your left-most carbon atoms are at y=-4.9, but the carbon atoms at the
other edge are at y=7.3.
Btw. the x-positions of the hydrogen atoms don't seem reasonable as well.
Maybe that is the reason for your wrong band structures?
Cheers
Roland
[email protected] schrieb:
Thank you, Marcos, for such a quick response.
You are absolutely right about the low level of mesh cutoff that I used, I
used it on purpose to cut down on the calculation time. But there is a
thing that I still don't understand in your explanations: how may any
k-point sampling be related with calculation of dipole moment of my
system? Dipole moment is invariant (with respect of the used system of
coordinates) for electrically neutral system, therefore it should not
depend on the lattice vector determination and, consequently, on any
k-point sampling.
Is it?. Moreover, band structure calculations and dipole moment are
separated things (if only they are not connected somehow through
plane-wave method). So, I can understand why the magnitude of non-zero
dipole moment may vary depending on the mesh cutoff value, but zero should
be equal zero.
The question about dipole moment is important for me because I look for
the reasons which result in errors of my band structure calculation for
graphene ribbon. (Initially I thought that there was something wrong with
my input file information or something like this).
Thank you once again, hope you will help me with that weird thing.
Artem Baskin,
PhD student,
University of Illinois at Chicago
Artem,
Two things could be hindering your calculation. First, your mesh
cutoff is quite low. I would use at least 200 Ry to get only decent
results. In my experience with 300 Ry you get pretty good results for
a graphitic system. Second, I don't see any k-point sampling along the
ribbon length. This definitely could be the cause of your fluctuating
dipole value.
Cheers,
Marcos
On Sat, Mar 13, 2010 at 3:08 AM, [email protected] <[email protected]> wrote:
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
