Re: Variation of results in iterations

From: Robert Brunner (rbrunner_at_uiuc.edu)
Date: Mon Dec 06 2004 - 21:15:00 CST

In addition to these factors, running in parallel can result in
slightly different results due to changing the order of arithmetic
operations affecting round-off error.

Running on 1 processor, with the same random seed should be
deterministic.

Robert

On Dec 6, 2004, at 7:01 PM, Jérôme Hénin wrote:

> Hi,
>
> As far as I know, two runs using exactly the same simulation
> parameters and
> starting point, run on the same machine, and without Langevin dynamics
> (or
> specifying the seed for the pseudo-random generator), should behave
> exactly
> the same way, because the machine itself is supposed to be
> deterministic.
>
> If any of the above criteria is not met, the two simulations are
> likely to
> differ very slightly after a few hundred steps, which is to say that
> they may
> be wildly different after an average-sized run.
>
> Does something in your experience go against this? If yes, I would be
> interested to read more about it.
>
> Jerome
>
> Le mardi 07 Décembre 2004 00:25, vous avez écrit :
>> NAMD Friends,
>> I have a question.
>>
>> When we run simulations(optimisation or MD) for any
>> system under identical condition three times, we end up with results
>> which are not exactly same but
>> vary within a range, which we denote by error bar in the plot. My
>> question is inspite of everything remaining same, what could be the
>> possible reasons giving rise to
>> these differences in results? Is it because of computation error ?
>> or
>> anywhere in calculation in NAMD any other parameters involved which
>> causes so.
>>
>> Any suggestions and references highly appreciated.
>>
>> Pijush Ghosh.
>
> --
> Jérôme Hénin
> Equipe de Dynamique des Assemblages Membranaires
> Université Henri Poincaré / CNRS UMR 7565
> B.P. 239 54506 Vandoeuvre-lès-Nancy Cedex
> Tel : (33) 3 83 68 43 95 Fax : (33) 3 83 68 43 71
> http://www.edam.uhp-nancy.fr/


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