On 06/24/2011 11:14 PM, Jan Sommer wrote:
Hello again,
I have still some problems with plotting bandstructures.
In an easy example I calculate the bands of a (9,0)-cnt. Thanks to you I
understood the width of the k-points right.
I than doubled the length of the cnt (so the siesta-unit-cell contains 2
cnt-unit-cells).
The width of the x-axis is now half of the former as expected by a
doubled lattice constant.
What bothered me is that the bandstructure of the cnt with length 2 is
folded somehow.
For a bandstructure calculation, you should always fully relaxation your
system to its equilibrium geometry firstly. When you double the length
of your cnt along its axis direction, though this is the same with your
orignal cnt according to periodic boundary conditions, I think the
geometry optimization should be done carefully before the bandstructure
calculation, in addition, the vacuum thickness in other two directions
perpendicular to the axis should also be retested, I think. As for the
others things, the k-points samping density should be the same, and fro
the other convergence criteria shoulbe also be tested carefully.
Strictly speaking, cnt is a quasi-one-dimensional periodic structure,
not a perfect three-dimensional periodic structure. So when change the
length of it, I think we must firstly ensure the image interaction
between itself should be fully eliminated.
In the attached file I made a quick and dirty plot. The red plot
corresponds to the cnt with length 1, the green with length 2. I marked
some points to show the mentioned folding.
My question is, if there is a way to unfold the green plot, so that the
green and red plot would be identical?
I don't think so, when you double the length of cnt in its axis
direction, the number of eigenstates should also be doubled, as a
result, there will be more bands appeared in the bandstructure plot.
I think the more important thing in your case is to ensure your do a
sensible and rational calculation first.
In my further research it would be necessary to be able to plot
bandstructures of the same cnt but different lengths together.
Why do this? By saying the same cnt with different length, do you mean
you have applied a strain along its axis direction? If you keep the
same cnt under the same external conditions ( strain, pressure,
temperature, and so on ), I think this no need to do a series of such
calculations you mentioned here, instead, just calculate the
bandstructure of cnt including appropriate primitive unit cells in its
axis direction will give you the understanding of the physical
properties of it.
HTH.
Regards.
This way,
I could use as short unit cells as possible and save computing time.
Thanks for any help.
Best Regards,
Jan
Zitat von Miguel Eduardo Cifuentes Quintal <[email protected]>:
Well, if I'm right, the internal units of siesta and others DFT-codes
are the atomics ones, but for this specific case you can take a look
at the bands.F file in the source directory of the siesta code, and
confirm this by yourself.
Some time ago also I was wondering about the units of this axis and I
tried with different possibilities. So my suggestion is to compute by
hand the coordinates on atomic units of two points of one of your
bandlines path, calculate its distance and then just divide by the
number of points of this bandline and finally, compare with the x-axis
of your band structure file. Also you can employ the systemlabel.KP
file (in atomic units!) in order to identify high symmetry points and
save some math :)
Best regards,
Eduardo Cifuentes.
-------------------------------------------------------
Miguel Eduardo Cifuentes Quintal
PhD Student
Department of Applied Physics
Cinvestav-Merida, Mexico.
e-mail: [email protected]
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Miguel Eduardo Cifuentes Quintal
PhD Student
Department of Applied Physics
Cinvestav-Merida, Mexico.
e-mail: [email protected]
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--
Hongyi Zhao <[email protected]>
Institute of Semiconductors, Chinese Academy of Sciences
GnuPG DSA: 0xD108493
