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Re: [lammps-users] Infused Rigid Particles moves diagonally
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Re: [lammps-users] Infused Rigid Particles moves diagonally

From: Axel Kohlmeyer <akohlmey@...24...>
Date: Wed, 5 Jul 2017 07:28:15 -0400

On Wed, Jul 5, 2017 at 12:50 AM, rofiques salehin <rsalehin92@...36.....24...> wrote:
Hello All,

I am running MD a simulation where rigid particles moves in srd particle due to applied strain. Although I set the velocity 0.0 in all direction , the following npt command updates the velocity of the rigid big particles such a way that in movie it moves in diagonal direction !

Why this is happening ? Thanks in Advance.

​there is not much helpful information here to go on and provide specific advice, but ​two things stand out:

1) it sounds like you are confusing "rigid" and "immobile". a rigid object is where a group of particles move as a whole, an immobile object is where particles do not move at all. in LAMMPS, rigid objects need a special fix to update the positions accordingly, and particles can be immobilized by either excluding them from time integration or setting them up, so their velocity remains zero.

2) the velocity command is a one-time action and applied immediately. if after that, a particle feels a force and is time integrated it will accelerate (again). that are newton's laws of motion in action. to keep such time integrated particles immobile, you also have to zero out their forces, e.g. using fix setforce. when using a thermostat, please also consider the impact of such operations on the temperature of other atoms in a group that includes immobilized atoms.


compute csym all centro/atom fcc
compute peratom all pe/atom 
fix 5 all npt temp 300 300 1 iso 0 0 1 drag 1 
thermo 100
thermo_style custom step lx ly lz press pxx pyy pzz pe temp
run 1000
unfix 5
variable tmp equal "lx"
variable L0 equal ${tmp}
print "Initial Length, L0: ${L0}"
variable srate1 equal 0.01
fix 6 all deform 1 x erate ${srate1} y volume units box remap v


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Dr. Axel Kohlmeyer  akohlmey@...12...24...
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.