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Re: [lammps-users] Different outcomes with same *.data & *.in while changing the parallel core numbers
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Re: [lammps-users] Different outcomes with same *.data & *.in while changing the parallel core numbers


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Sun, 11 Jun 2017 01:27:41 -0400



On Sun, Jun 11, 2017 at 12:13 AM, 高瞻 <sandy950111@...24...> wrote:
Hello everyone,
       I use the TIP4P-2005 water model to simulate the two-phase process of water, but when I change the parallel core number from 100 to 20, the outcome of the simulation changes a lot.

​what do you mean by "outcome changes a lot"? is it just different positions of individual atoms, or significant differences in (averaged) properties?

also, *always* mention the LAMMPS version you are using and best provide your input file, too, so that people can check, whether there are obvious mistakes somewhere.
 
       I can't understand why, should I change the simulation parameters such as the timestep while I set it as 2fs now or any other parameters to solve this problem?

​diverging trajectories with different number of cores, even different implementations of the same potential, are quite normal, as LAMMPS employs floating point math and floating point math does not associate (e.g. the exact outcome can be slightly different for all properties that​ are summed up in different order) which will result in an exponentially growing difference between such runs.

so at this moment, it is not even clear, whether you are reporting a real issue, or just physics and numerical science at work.

axel.

 

Thanks a lot!

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--
Dr. Axel Kohlmeyer  akohlmey@...12...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.