[Pw_forum] big calculations

[Nicola Marzari]

Given your system size, it's likely that gamma point
is good enough - a full relaxation with 18 Ry cutoff and
the gamma point would bring you very very close to the correct
result.

Then, try 30 Ry and Gamma point - do the results change a lot ?
Probably note, and maybe 25 Ry is enough (there are tests of
the C ultrasoft on the pseudopotential page). I'd assume a dual of
8, i.e. a cutoff for the charge density of 8 times the cutoff on the
wfcs.

The 2 2 2 1 1 1 mesh will be almost certainly more than enough
(i.e. a 2x2x2 shifted) -  you won't need more than these, and probably 
gamma is enough.

Note that if you use the keyword gamma for the k-point mesh
pw takes into account that at gamma wavefunctions are real,
halving the memory and speed.

Last, my favourite trick is to use 1/4 1/4 1/4 as a k-point
(in relative coordinate), and stop pw from replicating that
with nosym=true . In this way, you use only one k-point
(albeit with complex wavefunctions) and the results are almost
indistinguishable from the 2 2 2 1 1 1 mesh.

For even larger systems, it might be worth becoming familiar with
cp, and use a combined damped dynamics on electrons and ions
(actually, only on the electrons tend to be enough). This can
be quite efficient for huge systems, and faster/smaller calculations
than pw.

At a certain point we shoudl write a small tutorial about that...

			nicola


On 1/19/11 8:58 PM, José Alberto Pires Fernandes wrote:
> Hello
>
> I'm a newby in calculations with Quantum espresso and I
> think I'm trying to do an impossible task.
>
> I want to do a structural optimization to a cell with 338
> atoms (C, H, N and O) and approximate dimensions of
> 15*15*15  angstroms. If I choose Ecut of 18 Ry and a
> kpoint mesh of 1*1*1, with pz-rrkjus pseudopotential the
> optimisation takes 13 hours. With a 2*2*2
> kpoint mesh is taking more than 4 days, and hasn't
> finished yet. I believe that for reliable results I need a
> larger Ecut (at least 30 Ry, I suppose) and a number of
> kpoints in the order of thousands, which would imply in a
> calculation of several years.
>
> The cell is a P1 triclinic cell but about 300 atoms are
> fixed, because I only need to do particial optimisation.
> So, what can I do to have reliable results in useful time,
> or Quantum Espresso is not suitable for my task?
>
>
>
>
> José Fernandes
> Departamento de Quimica
> Universidade de Aveiro
> 3810-193 Aveiro
> Portugal
> Tel: +351234370720
> Fax: +351234370084
> _______________________________________________
> Pw_forum mailing list
> Pw_forum at pwscf.org
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-- 
----------------------------------------------------------------------
Prof Nicola Marzari    Department of Materials    University of Oxford
Chair of Materials Modelling  Director, Materials Modelling Laboratory
nicola.marzari at materials.ox.ac.uk     http://mml.materials.ox.ac.uk/NM