[Pw_forum] (no subject)

Lorenzo Paulatto paulatto at sissa.it
Thu Nov 19 08:20:05 CET 2009

On Wed, November 18, 2009 19:33, zahra sadat naghavi wrote:
>  Could you please tell me is it possible to solve a system with pwscf that
>  is not periodic?

Dear Zahra,
pw.x always uses periodic boundary conditions, it is possible to simulate
an isolated non-periodic system by putting it in the middle of a big cell,
so that periodic images don't interact. There are also a number of
algorithms to threat this case more effectively that can be activated with
input parameters.

>  For example, I would like to calculate phonon frequencies of a molecule
>  with a larg amount of atoms such as 500,defining this molecule
>  in a periodic lattice is easy but its phonon caculation is impossilble
>  here.

500 atoms are a lot for plane-wave based codes, strictly speaking what
matters is the number of electrons, but unless they are all Hydrogens it
does not make a big difference. You definitely can, but it will take much
longer than you expect; I would not even bother trying with less than 256
processors *available all time*. You can have a look at this page
where the scaling with system size is explained in detail.

>  Does anybody have any solution?

To be honest, I would go for a localized basis set code.

best tregards

Lorenzo Paulatto
phone: +39 040 3787 511
skype: paulatz
www:   http://people.sissa.it/~paulatto/

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