[CP2K:8664] Re: Comparison between CP2K and Quantum Espresso

[Ari Paavo Seitsonen]

Dear Tommaso and David,

  I would definitely _not_ call using Vanderbilt pseudo potentials as
having "minimal basis set", usually the precision is better than with
norm-conserving pseudo potentials (due to several reference
energies/projectors). The reduction of cut-off is due to a good reason. So
lowering the basis set in CP2K would be unfair, as the results are of lower
precision. Rather using GAPW (instead of GPW) would have kind-of the same
effect, as this would allow a lower cut-off energy being used in the plane
wave part of QuickStep.

  The idea of shifting the k point might refer to the scheme of
Baldereschi's special point (I do not know much, I was just told that this
is the "best" point if using a single point; since most people do not know
it and Monkhorst-Pack is so "black box"-convenient, younger generations
have probably not become users of it). Just my guess.

    Greetings from Paris,

       apsi

2017-02-08 11:28 GMT+01:00 <tfran... at gmail.com>:

> Dear David,
> first of all thanks a lot for your very useful comments.
>
> I will benchmark my system in CP2K with a lower basis sets, in order to
> test again the parameters and to have better reference with respect to QE.
>
> Regarding the compilation of the program in our cluster, I think that it
> is the pitfall and that our technician missed something. He told me (
> because I am not allowed to install anything, but only the technician )
> that he compiled the precompiled version of CP2K. But I am pretty sure he
> missed some of the libraries you listed above. Indeed I am insisting in a
> more "serious" compilation of the program, that we wanna massively exploit
> for our dynamics.
>
> Regarding the single point shift from the gamma point in QE, that was
> suggested by one of my old Professors. At that time ( almost three years
> ago) he told us that the shift is recommended.
> I always used that setting, without honestly taking care too much about
> the change in performances. By the way, thanks a lot for the really useful
> hints. Now I will re-benchemark the new machine testing several levels of
> theory as you suggested.
>
> Best Regards,
> Tommaso Francese
>
> Il giorno mercoledì 8 febbraio 2017 11:09:16 UTC+1, David T ha scritto:
>>
>> Hi Tommaso
>>
>> as you I am more expert on QE and only recently moved on CP2K.
>> On my experience CP2K is much more quicker and allows to work with bigger
>> systems that planewave code could not afford. On the other hand it is true
>> that this is really system depended so the fact that for nano-porous
>> material CP2K is more efficient could not be true for other systems.
>> After having read the inputs a comment I can give to you is that you are
>> probably not making a fair comparison.
>> I've see you use ultrasoft pseudo which allows a very low cut-off and can
>> be "though" as minimal basis-set. So you should probably use some
>> corresponding low basis-set in CP2K (for instance a dobule zera instead of
>> triple zeta).
>>
>> Probably more important another thing I can tell you is that I've seen
>> that if CP2K is not properly compiled, its performance can be slow. For
>> instance my own version with mpi, mkl, libxsmm and elpa is about 30% faster
>> than the standard one I found on our cluster.
>> Moreover the speediness of the code can be further boosted if you use
>> also GPU and hybrid openMP-MPI.
>>
>> P.S. a curiosity, in QE why are you using a single point shifted from
>> gamma? if there is not a major reason using the gamma algorithm on QE can
>> accelerate you calc up to 30%
>>
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-- 
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  Ari Paavo Seitsonen / Ari.P.S... at iki.fi / http://www.iki.fi/~apsi/
    Ecole Normale Supérieure (ENS), Département de Chimie, Paris
    Mobile (F) : +33 789 37 24 25    (CH) : +41 79 71 90 935
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