[Pw_forum] CNT unit cell vectors

[xirainbow]

Dear Mahmoud Hammouri:
           For Tetragonal P, "a" must be equal to "b". Therefore, it
is forbidden to set "b" as an individual variable.
           For an isolated CNT, you can set "a" arbitrarily. But the
condition is not true for "c" ("c" is along the axis of CNT ). The
length of "c" of CNT changes much with the chiral angle of CNT.


On Fri, Jun 14, 2013 at 5:22 PM, Mahmoud Hammouri <mhm at nmsu.edu> wrote:
> Hi all,
> I have problem for defining the lattice vectors of carbon nanotubes, I saw
> some examples for zigzag CNTs and they are using ibrav =6 with celldm(1)=31
> and celldm(3)= 0.25 and in another file celldm(3)=0.1704 I'm really wonder
> why they did not define celldm(2)? Now, for more than one unit cell how
> celldm(3) would change.
> In the descriptions of the pw.x input file, for Tetragonal P (st):
>  celldm(3)=c/a and since c=1.42 and a=4.26 then celldm(3)=0.333 not 0.25 or
> 0.17 as others said!!!
>
>
> Regards
>
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-- 
____________________________________
Hui Wang
School of physics, Fudan University, Shanghai, China