[Pw_forum] How many repeated units along the periodical direction should be used for nanowire/nanoribbon calculations?

[Hongsheng Zhao]

Hi all,

I've read some papers on the study of properties for 1D low dimensional 
nanostructures, such as nanowire/nanoribbon.

In all of these papers, they usually take more than one repeated units 
along the periodical direction to construct the

actual supercell for their calculations. As an example, the following 
paper: Uniaxial strain modulated band gap of ZnO nanostructures [APPLIED 
PHYSICS LETTERS 96, 213101, 2010] use the following settings for its 
calculation (see page 1 on that paper):


-------------
Test calculations indicate that two repeated units along the axis are 
required for ZnO NWs and
NTs.
-----------

For the paper mentioned here, you can download it from here:

http://h1.ripway.com/zhaohs/Uniaxial%20strain%20modulated%20band%20gap%20of%20ZnO%20nanostructures.pdf

My issue is: how can I determine the minimum repeated units along the periodical direction should be used for nanowire/nanoribbon calculations?

For my issue, I think the decision-making process for the above probem is as follows:

1.  Do a series of convergence test w.r.t. MP grid, E_cutt and so on, based on the supercell which includes one repeated unit along the periodical direction.  By this way, we can find the convergence parameters used for  follow-up calculations.

2. Based on the convergence parameters obtained from step 1., do a geometry optimization calculation for the supercell included one repeated unit along the periodical direction and found the stable equilibrium structure for this supercell.

3. Based on all of the calculation parameters to obtain the stable equilibrium structure,  we change the numbers of repeated units along the periodical direction and do a series of single single point energy calculations for these supercells with different repeated units along the periodical direction.

4.  Finally, we calculate the total energy per repeated unit, i.e.,  E_total/[repeated unit] , and plot this energy with the number of repeated units and find the minimum repeated units which can ensure the E_total/[repeated unit] has a relative stable value.

Am I right?   Any hints/improvements for my above description will be  highly appreciated.  Thanks in advance.

Regards.
-- 
Hongsheng Zhao<zhaohscas at yahoo.com.cn>
School of Physics and Electrical Information Science,
Ningxia University, Yinchuan 750021, China

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