[Pw_forum] big calculations II

[Nicola Marzari]

On 1/20/11 2:26 AM, 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


Dear Ricardo,

two options, I think - the simplest is that a nscf calculation,
dealing with one k-point at a time, would not really require more
RAM, but for the fact that k-points different from gamma have
complex wavefunctions (so indeed you might be out of luck).

Another possibility would be to calculate the Wannier functions
for this system, and then use Wannier interpolation (see our 2005
PRL Lee et al.). This can work extremely well, but it requires learning
a new code.

As a side note, the CP codes requires much less memory than PWSCF -
it has only gamma sampling, though. It does deal with metals, using
the "ensemble-DFT" approach, but that has not been optimized as much
as the other parts. Worth trying, though!

A last resort would be to decrease the thickness of your slab - (111)
surfaces could be ok with as little as 3 layers, or maybe even 2 (maybe 
you could put a few more atoms in the 3rd layer below the cobalt).

			nicola



-- 
----------------------------------------------------------------------
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