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 Jos?. 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 F?sica 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 > escribi?: > > > 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, Jos? 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? >> >> >> >> >> Jos? 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 > Pw_forum at pwscf.org > http://www.democritos.it/mailman/listinfo/pw_forum
