[Pw_forum] Relaxed Graphene Nanoribbon

[Giovanni Cantele]

On May 9, 2011, at 5:36 AM, swapnil chandratre wrote:

> Hi, I have prepared an input file for a porous graphene nanoribbon, can anyone please comment if there are any errors with the file.

it is always a good idea to open the input file using XCrysDen (or any other visualization software with a working Quantum-ESPRESSO interface),
because this often helps in pointing out "structural" (cell paramenters and/or atomic positions) issues (see below).

> ecutwfc=80.0
> ecutrho = 600.0
.....
> C 12.0000 C.pz-rrkjus.UPF
> H 1.00000 H.pz-rrkjus.UPF
> 
because your pseudos are ultrasoft a 80 Ry cutoff is likely very huge,
you can reach convergence with much lower values, I think.


Concerning the structural part, there is a problem for sure.
> CELL_PARAMETERS
> 19.200000 0.000000 0.000000 
> 0.0000000 21.90350 0.000000
> 0.0000000 0.000000 10.00000

you chose an orthorombic unit cell with axis lengths of
about 10, 11.4 and 5,0 Angstrom, respectively. 

But if you look at the atomic positions,
you can easily see that:
>   C   0.710430  9.644997  5.000000

>   H   20.44254  5.000000  5.000000

the x coordinates of these two atoms differ by 20.4 - 0.7 = 19.7 A which is less than your unit cell size along the x axis.
The mistake probably originates from the fact that if you DO NOT specify celldm(1), the units of coordinates within the CELL_PARAMETERS card
is a.u., namely, Bohr. I guess that you want to specify those units in Angstrom, indeed, the structure looks much better (with a hole that is what you mean with "porous") if you set celldm(1)=1.889727
(inverse of Bohr). Anyway, orthorombic cells are much easier specified by setting ibrav=8 and then a, b, c as axis lengths in Angstrom (see Doc/INPUT_PW.txt).

Even after correctly specifying axis length units there is still a problem with the above atoms, because the 19.7 A difference of x coordinates is still larger than the 19.2 Angstrom a-axis. So, either
the atomic positions are wrong, or the cell size was incorrectly computed.

Giovanni





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Giovanni Cantele, PhD
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