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

[Axel Kohlmeyer <akohlmey@...24...>]

On Tue, Jul 4, 2017 at 1:39 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 20 to 100, the outcome of
> the simulation changes a lot. My LAMMPS version is lammps-23Jun17, and my
> input file is as follows:
>
> # variables input
>
> variable         DT equal 1.0
> variable         A equal 220
> variable         r equal 10
> variable         L equal 75
> variable         THERMO equal 500
> variable         Pstart equal -0.5*10*$A
> variable         Pstop equal -0.5*10*$A
>
> # init
>
> units            real
> dimension        3
> boundary         p p p
> atom_style       full
> timestep         ${DT}
> neighbor         2.0 bin
> neigh_modify     delay 0 every 1 check yes
>
> # create geometry
>
> pair_style       lj/long/tip4p/long long long 2 1 1 1 0.1546 9 8.5
> kspace_style     pppm/disp/tip4p 1e-5
> kspace_modify    force/disp/real 0.0001 force/disp/kspace 0.002 mix/disp
> geom
> bond_style       harmonic
> angle_style      harmonic
> read_data        data.droplet_npt
>
> # variables output
>
> variable         PRESS equal press
> variable         VOL equal vol
>
> # fixes
>
> fix              0 all indent 10.0 sphere $L $L $L $r units box
> fix              1 all shake 0.0001 100 0 b 1 a 1
> fix              barostat all npt temp 298.0 298.0 $(100.0*dt) iso ${Pstart}
> ${Pstop} $(1000.0*dt) drag 1.0
> fix              print_press all print 1 "${PRESS}" file press.dat  screen
> no title ""
> fix              print_vol all print 1 "${VOL}" file vol.dat screen no title
> ""
>
> # run
>
> thermo_style     custom step temp press vol
> thermo           ${THERMO}
> run              200000
>
> The simulation is designed to mimic the cavitation in water at a low
> pressure, and the "outcome changes a lot" means the vol-time curves in the
> simulations are different. The vol variable in the 20 core simulation is
> increasing much quicker than in the 100 core simulation, which makes me
> confused.

...and with 4 cores, it doesn't even run. it will stop after a few steps with:
ERROR on proc 1: Out of range atoms - cannot compute PPPM
(../pppm_disp_tip4p.cpp:111)
application called MPI_Abort(MPI_COMM_WORLD, 1) - process 1

this usually happens, when you have some unphysical forces in your
system. it looks like those are cause by your indenter.
your volume curves don't make much sense to me either. both look like
your system is rapidly expanding, like in an explosion or evaporation.
that is unphysical, too, so it could be caused by the indenter as
well.

it is not clear from your description what the purpose of the indenter
is in the first place. if you want to see a cavity, you can just set
up a system with a suitably low density at fixed volume, and the
vacuum bubble for by itself.

axel.

> What I want to ask for advice is that is there any other factors except the
> "floating point error" that cause this situation? Because I think the the
> "outcome change" in the two simulation is really beyond my expectation.
> The in script is as attachment, and the vol-time plots are as attachments
> too.
> The data file is sent to Axel directly because it is too big.
>
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-- 
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.