Am 25.06.2011, 11:20 Uhr, schrieb Hongyi Zhao <[email protected]>:
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.
These thinks are clear. The mentioned thing was just a quick and dirty
example as a calculation only runs a few minutes.
The normal cnts I use have mostly more than 100 atoms in their unit cell.
Why do this? By saying the same cnt with different length, do you mean
you have applied a strain along its axis direction?
I mean how much CNT-Unit-Cells are explicitliy in the siesta input file.
In the example 36 or 72 (corresponding to 1 or 2 unit cells of the
(9,0)-cnt)
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.
That's true if I would be interested in only the bare cnt.
In future I want to dope the cnts or alter them in general. So in some of
these alterings the primitive unit cell would correspond to a cnt of
"length 1" (36 atoms in the above example), some to one of "length 2" (72
atoms) and a few maybe even to "length 3".
As I want to be able to compare them together in a joint plot, I could
always multiplicate the smaller systems to the size of the longest so the
brillouin zone always had the same size, but this would result in a big
overhead in computing time, especially as relaxation can take very long. I
just want to make sure beforehand that these things work. That's why I
also use a quite small cnt at the moment, so extra calculations to test
things don't last long.
If you take a look at the image of my last email, you see that the bands
are basically the same. The thing is, that the right and left side of the
red plot are folded inwards in the green plot and I dont really know how
to determine the corresponding points in the data file to unfold them.
I hope, I could make my problem clearer this time.
Maybe as it is a new and different question I should open a new thread...?
But thank you all very much for your efforts in helping me out.
Best Regards,
Jan
