[Pw_forum] big calculations II

Thu Jan 20 06:43:56 CET 2011

> to your recent PRL). I optimized the structure with gamma point with
> 25 and 250 ecut and rhocut respectively

Only a little suggestion (I've performed some similar calculations): I suppose 
that your Co pseudopotential is quite far from convergence at 25 Ry... If 
your calculation of the isolated molecule at gamma is converged at 25 Ry 
PLEASE SHARE THE PSEUDOPOTENTIAL...:-) !!!! I suppose also that forces 
calculated by using a higher pw cutoff and a well converged k-point mesh on 
the gamma/25 Ry geometry (molecule+surface, of course) will be still quite 
high. Perhaps you should consider to migrate your calculation to a better 
machine...
HTH

Yours

Giuseppe Mattioli

On Thursday 20 January 2011 03:26:27 Ricardo Faccio wrote:
> Dear nicola
> Thanks gr your email, i would like to take the opportunity and ask you
> a further question. I have a similar situation than José. I'm working
> with a cobalt porphyrin deposited over a metallic substrate (similar
> to your recent PRL). I optimized the structure with gamma point with
> 25 and 250 ecut and rhocut respectively. It takes almost the 100% of
> my 8Gb RAM quad core computer. Now i want to get a reasonably DOS fir
> this system, but i need more k-points, and thus I will need more RAM
> memory. Would you recommend anything else for getting a good DOS for
> my quad core 8 GB RAM machine?
> Thank you in advance.
> Ricardo
>
> --------------------------------------------
> Dr. Ricardo Faccio
> Prof. Adj. de Física
>
> Av. Gral. Flores 2124. CC 1157. CP 11800.
> Phone: + 598 2 924 9859
> Fax: + 598 2 924 1906
> Web: http://cryssmat.fq.edu.uy/ricardo/ricardo.htm
> ---------------------------------------------
>
> El 19/01/2011, a las 21:54, Nicola Marzari
> <nicola.marzari at materials.ox.ac.uk
>
>  > escribió:
> >
> > 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
> >> http://www.democritos.it/mailman/listinfo/pw_forum
> >
> > --
> > ----------------------------------------------------------------------
> > 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
> > _______________________________________________
> > Pw_forum mailing list
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> > http://www.democritos.it/mailman/listinfo/pw_forum
>
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