[Pw_forum] 1D systems

[Stefano Baroni]

Dear Michele,

I think what you are doing is (generally speaking) correct. Having the two in-plane lattice vectors larger than the diameter of the tube would allow for some "vacuum" between neighboring periodic images of our tube, so as to simulate to some extent an isolated tube. This is what is called the "supercell" technique. Note that the physical parameter to monitor is the ratio between the distance between neighboring tubes (i.e. the length of the in-plane lattice vectors) and the diameter of the tube, *NOT* the length of the lattice vector parallel to the cube.

As a general rule, you should check the convergence of the physical quantity you are interested in (be it the total energy, some orbital energies, some other physical properties ...) with respect to the inter-tube distance. The total energy, in particular, *HAS* to converge. If it does not, this means that you are doing something different from what you have described (and, probably, from what you think you are doing ...).

If 1 day of careful inspection of the input data and of the results, plus discussions with your peers and supervisor(s) does not produce any results, you may want to consider reverting to us with some more details on what is going on ...

HTH - Stefano B

On Dec 14, 2011, at 2:07 PM, Michele Pisarra wrote:

> Dear QE users,
> I used QE to calculate the wave functins of several types of carbon nanotubes. To simulate the 1D nature of this system I used cell parameters such that two of the lattice displacement vectors are much larger than the other one. However this choice lead to a reciprocal space with two very small vectors and so to a very large number of plane waves in the basis set, ecut being fixed. I need to reduce the number of plane waves as much as possible. I tried to make different runs using different lengths for the two "irrelevant" vectors and I found that decreasing this lenght decreases the total energy. However I don't think that the total energy is the good parameter to look at in my case, because the decreasing of the total energy can be due to the interaction between the replica of the system that I want to be zero. So I need a criterion to find the best configuration. Can anyone help me?
> Thank you.
>  
> Michele Pisarra, PhD Student,
> Dipartimento di Fisica, Università della Calabria
> Via P. Bucci, Cubo 31 C VI piano, 87036, Rende (CS), Italy
>  
> 
> 
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---
Stefano Baroni - SISSA  &  DEMOCRITOS National Simulation Center - Trieste
http://stefano.baroni.me [+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)

La morale est une logique de l'action comme la logique est une morale de la pensée - Jean Piaget

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