[Q-e-developers] system size

[Laalitha Liyanage]

Many thanks to Emine and Paolo for correcting me.
I was under the impression that DFT could handle only a few hundred atoms.
It seems I am mistaken. I hope to try to run one of these large systems 
in the future

Laalitha Liyanage
Department of Physics
Central Michigan University
Department of Physics
University of North Texas

On 04/25/2013 03:57 PM, Kucukbenli Emine wrote:
> Dear Milinda, Laalitha
>
> This question actually belongs to pw_forum mailing list (subscribe 
> here : http://pwscf.org/mailman/listinfo/pw_forum )
>
> Let me answer from experience,
> Some years ago I have run calculations on a system of >1000 atoms of C 
> H O N atoms (cholesterol molecular crystal)
> from as little as 128 to as many as  1024 processors on an IBM SP6 
> machine and also on a Linux cluster.
>
> I never had any problems with memory while running an optimization 
> calculation, I dont think it ever exceeded 1.5Gb per processor.
>
> I have just checked one of these calculations for you, 176 processors, 
> the code prints as the largest occupied per processor dynamical memory 
> as 961 Mb.
>
> Note that back then we didnt have band parallelization.
> I was also running my calculations with a very high plane wave cutoff 
> for increased accuracy (required for NMR calculations I wanted to run 
> afterwards) meaning that you can probably get away with less if you 
> only want to optimize atomic positions.
>
> Of course you can always use a less accurate but cheaper code first 
> and fine tune your findings with Quantum Espresso later, but I found 
> it was not necessary in my case, starting from experimental positions 
> were enough.
>
> best,
> emine kucukbenli, postdoc at theos, epfl, switzerland
>
>
>
> ________________________________________
> From: q-e-developers-bounces at qe-forge.org 
> [q-e-developers-bounces at qe-forge.org] on behalf of Laalitha Liyanage 
> [laalitha.liyanage at unt.edu]
> Sent: Thursday, April 25, 2013 10:14 PM
> To: Milinda Samaraweera; General discussion list for Quantum ESPRESSO 
> developers
> Subject: Re: [Q-e-developers] system size
>
> Dear Milinda,
>
> 792 atom system is too large for Quantum Espresso.
> You can try to use SIESTA http://icmab.cat/leem/siesta/
>
> Laalitha Liyanage
> Department of Physics
> Central Michigan University
> Department of Physics
> University of North Texas
>
> On 04/25/2013 01:01 PM, Milinda Samaraweera wrote:
> > Milinda Samaraweera
> > Computational materials chemistry and Computer programming
> > University of Connecticut
>
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