[Pw_forum] How to create a supercell

[stefano de gironcoli]

Dear Sapna Bondwal,

pw.x (almost**) always describes systems that are periodic so if you 
desire to approximately describe an isolated cluster you need to 
surround it by some vacuum space so that periodically repeated images 
interact as little as possible.

let's say you want to describe SiH4, let's say SiH bondlength is about 
1.5 AA, so the molecule is about spherical with a diameter twice as 
much, 3 AA... add 6-7 AA of vacuum to separate it from its periodic 
image and you get ~20 a.u.: a SC cell of that size should be reasonable.

the easiest way to specify the coordinates is to choose ATOMIC_POSITIONS 
in angstrom or bohr so you don't have to bother about rescaling them 
when you test convergence w.r.t. cell size... something that you should 
always do to make sure your cluster is really isolated.

**Once you are at ease with this simple calculations for isolated 
molecules you can explore the
effect of the assume_isolated flag (there are various options that 
correct for the spurious interactions among periodical images with 
various degrees of sophistication)

stefano


On 11/10/2015 14:30, sapna bondwal wrote:
> Hello,
>
> I am working on Silicon nanoclusters. A nanocluster is not a periodic, 
> so one needs to create artificial periodicity using supercell. I 
> choose say, ibrave = 1 i.e cubic lattice, cell dimensions= 20.00. 
> Could someone guide me how to create a supercell using these 
> parameters? How to give coordinated of a cluster for example Si5 or 
> SiH4? How to specify the vacuum space, Cell length? Could someone give 
> me clear idea about my query? What is the importance of supercell 
> approach? I request someone to give a simple illustartion so that I am 
> able to appreciate the supercell approach in a better way.
>
> Thank You in advance.
>
> -- 
>
> Sapna Bondwal
> Research Scholar
> Theoretical Chemistry Lab
> IIT ROORKEE
>
>
>
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