[Pw_forum] Doubt in computation time

[Nicola Marzari]

Dear Neel,

there is no difference. You can specify the size of your unit cell using 
different units of measure, but provided you
convert your units correctly, you will of course have the same unit 
cell, and same computational cost.

Of course you can use larger cells that contain multiples of the 
primitive unit cell, and this a larger "nat"
and a larger computational cost.

Suggestion - before proceeding further with the code, try to study 
silicon using a 2-atom fcc unit cell,
specifying atomic positions in any of the possible modes, and checking 
that the results and the computational
time does not change.

                nicola



Neel Singh wrote:
> HI 2 all,
>
> I have a doubt. Please help me, i will be very thankful to all of you. 
> When we define our system in terms of atomic positions, we have four 
> option like bohr, angstrom etc. I want to know weather the 
> computational time will be less when we describe any crystal structure 
> in option crystal (atomic positions {crystal}) as compared to 
> angstrom/bohr(atomic positions {angstrom}) as the parameters nat 
> (number of atoms) in &system will less in crystal option as compared 
> to angstrom/bohr option.
>
> Thanks
>
> Neel Singh
> University of Delhi,
> India
>
>
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Prof Nicola Marzari   Department of Materials Science and Engineering
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