[Pw_forum] A "relax" input runs on CPU (pw.x) but not on CPU-GPU (pw-gpu.x)

[Axel Kohlmeyer]

On Fri, Jun 20, 2014 at 4:22 AM, Reza Behjatmanesh-Ardakani
<reza_b_m_a at yahoo.com> wrote:
> Dear Filippo
>
> Due to the nature of QE which is iterative, I think lack of ECC and even double precision floating point in gaming cards (GTX) comparing to tesla cards
>
> is not serious problem for QE-GPU. Some authors have checked this for AMBER molecular dynamics simulation code. See following site:

classical MD is a very different animal than what you do with QE.
errors due to single precision to some properties in classical MD are
huge with all single precision calculations. to compute a force from a
distance will not be much affected, but summing up the force can
already be a problem. "good" classical MD codes usually employ a mixed
precision approach, where only the accuracy insensitive parts are done
in single precision. for very large system, even double precision can
show significant floating point truncation errors. usually you are
dependent on error cancellation, too, i.e. when you study a simple
homogenous system (as it is quite common in those tests).


>
> http://www.hpcwire.com/2014/03/13/ecc-performance-price-worth-gpus
>
>
> and see the following paper:
>
>
> www.rosswalker.co.uk/papers/2014_03_ECC_AMBER_Paper_10.1002_cpe.3232.pdf
>
>
>
> I encourage the users of QE-GPU to test it for QE, and report the difference on the site.

it is a waste of time and effort. people have done DFT and HF in
(partial) single precision before and only if you write a new code
from scratch and have an extremely skilled programmer, you will
succeed. have a look at the terachem software out of the group of todd
martinez for example.

axel.

> PS: to be able to test the results for GTX and TESLA, it is needed QE-GPU code to be run on GTX :-)
>
>
> With the Best Regards
>
>    Reza Behjatmanesh-Ardakani
>    Associate Professor of Physical Chemistry
>    Address:
>    Department of Chemistry,
>    School of Science,
>    Payame Noor University (PNU),
>    Ardakan,
>    Yazd,
>    Iran.
>    E-mails:
>           1- reza_b_m_a at yahoo.com (preferred),
>           2- behjatmanesh at pnu.ac.ir,
>           3- reza.b.m.a at gmail.com.
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-- 
Dr. Axel Kohlmeyer  akohlmey at gmail.com  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.