[QE-users] Huge memory requirements for a scf calculation of a system consisting with 300+ atoms.

[Nicola Marzari]

Fully agree with this! If I can add, using the Baldereschi point (or, to 
make it simple, 1/4 1/4/ 1/4 in crystal coordinates), and nosym=.true. 
allows you to do a calculation with one k-point that is almost as 
accurate as using a 2x2x2 shifted monkhorst pack mesh (i.e. 2 2 2 1 1 
1), at a cost that is only twice as large as a Gamma only calculation.

If a metal, use 0.03 Ry of smearing (either m-p or m-v), and indeed you 
are good to go for a fast but quite accurate relaxation.

				nicola

On 27/05/2020 13:39, Giuseppe Mattioli wrote:
> 
> Dear all
> Just to add a bit of personal experience that might be useful to others. 
> Let's admit that many k-points are necessary to provide a good 
> description of the electronic properties of a given system, this is 
> generally true in the case of metal systems. This fact might not extend 
> to the potential, and very tiny differences might be found in final 
> structures optimized by using a coarser sampling of the Brillouin zone. 
> In the huge cell shared by Hongyi Zhao, I would start a geometry 
> optimization from a gamma-only simulation and then I would check if 
> forces on ions were low enough with a 2x2x1 mesh. If this was not the 
> case, I would fully optimize the system with the new mesh and go a step 
> ahead, and so on up to a decent convergence of the potential. Then I 
> would perform nscf calculations with increasing numbers of k points 
> followed by, e.g., dos.x post processing runs, to check the convergence 
> of the density of states (be careful, because AFAIK nscf runs overwrite 
> the results of the scf run). Of course, all of this depends on the 
> specific purpose of the calculation, but in my past experience with 
> molecules on metal surfaces this strategy saves a lot of time and 
> resources.
> HTH
> Giuseppe
> 
> Quoting Sebastian Hütter <sebastian.huetter at ovgu.de>:
> 
>> Hi,
>>
>> This may be a stupid question, but...
>>
>>>      Estimated static dynamical RAM per process >       3.32 GB
>>>      Estimated max dynamical RAM per process >      10.52 GB
>>>      Estimated total dynamical RAM >     462.96 GB
>>
>> ... is this not expected behavior? I'm not super experienced, so I 
>> just assumed it was.
>>
>> Your numbers pretty much match what I see in terms of "RAM per Cell 
>> volume" in metals with non-symmetric unit cells using PAW pseudos, if 
>> not less. Random example: 126 atoms, 63 k-points, ~1000 bands, 250³ 
>> dense grid FFT gives ~10GB per rank, for a total of 680GB with 64 
>> ranks. I actually plan node requests for our cluster based entirely on 
>> memory required, probably wasting CPU time along the way (4N*16C in 
>> the example above).
>>
>> Reasonable ke and charge cutoffs seem to blow up the memory 
>> requirements a lot. Of course multiplied by the number of bands...
>>
>>
>> Best,
>>
>> Sebastian
>>
>>
>> -- 
>> M.Sc. Sebastian Hütter
>> Otto-von-Guericke University Magdeburg
>> Institute of Materials and Joining Technology
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> 
> 
> 
> GIUSEPPE MATTIOLI
> CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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> E-mail: <giuseppe.mattioli at ism.cnr.it>
> 
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-- 
----------------------------------------------------------------------
Prof Nicola Marzari, Chair of Theory and Simulation of Materials, EPFL
Director, National Centre for Competence in Research NCCR MARVEL, EPFL
http://theossrv1.epfl.ch/Main/Contact http://nccr-marvel.ch/en/project