Issue while dumping angular velocity in a 2D simulation

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1

I am simulating a 2D system of spherical colloidal beads in LAMMPS. I have added a torque by using the ‘fix addtorque’ command. The particle is rotating in the simulation as I can see that during visualization. But while dumping the angular velocity it is coming to zero. So is it due to the problem with dump command script or with 2D system? How solve this issue…

Can anybody help me with this…

The input script for the simulation is here;-

dimension 2

units lj

boundary p p p

atom_style sphere

neighbor 1.0 bin

neigh_modify every 1 delay 10

read_data test_sphere.data

pair_style lj/cut 2.5

pair_modify shift yes

pair_coeff 1 1 1.0 1.0 1.122

############# warmup integration #############

minimize 1e-5 1e-5 10000 10000

velocity all ramp vx 0.5 5.0 x 0 150 units box

group sph type 1

fix 1 all nve

fix 2 all langevin 1.0 1.0 10.0 954297 omega yes

fix 2dmini all enforce2d # 2D Simulation

timestep 0.001

thermo 10

run 100000

################# main run ----- compute ###############

fix 6 sph addtorque 0.2 0.2 0.2

fix 8 all nvt/sphere temp 1.00 1.00 10.00 disc # 2D disc

fix 87 all langevin 1.0 1.0 10.0 924856

variable tx equal torque(sph,x)

variable ty equal torque(sph,y)

fix syst sph print 10 “{tx} {ty}” file torque_sphere_ep4.dat

variable wx equal omega(sph,x)

variable wy equal omega(sph,y)

fix systw sph print 10 “{wx} {wy}” file omeg1_sphere_ep4.dat

reset_timestep 0

timestep 0.001

fix 2d all enforce2d # 2D Simulation

thermo_style custom step temp pe etotal press vol epair ebond eangle ecoul

thermo 10

dump 48 sph custom 10 traj_sph_ep4.lammpstrj id type x y z ix iy iz

dump W sph custom 1 omega_ep4.dat id type omegax omegay omegaz

dump 2 sph xyz 10 sph_pos_ep4.xyz

dump 4 sph custom 10 sph_velo_ep4.dat id type vx vy vz

run 1000000

Thanks in advance

-Ligesh

2

     I am simulating a 2D system of spherical colloidal beads in LAMMPS. I have added a torque by using the 'fix addtorque' command. The particle is rotating in the simulation as I can see that during visualization. But while dumping the angular velocity it is coming to zero. So is it due to the problem with dump command script or with 2D system? How solve this issue..
Can anybody help me with this...

the problem is in your lack of understanding of what you are doing and
your lack of care (or understanding) when reading the documentation.
there is little that we can do here about it.

in particular:
- you are using atom style sphere, but there is nothing in your model
that requires it (save for fix nvt/sphere which will act as fix nvt,
since there is no per particle torque generated anywhere)
- fix addtorque act on the fix group and adds per-atom *forces* on
group members, that represents the desired torque. the torque you add
btw. is invalid for a 2d system
- you are applying time integration and thermostatting to the same
particles multiple times. this is *very* wrong and at least for the
time integration LAMMPS will warn loudly about it.
- since the torque you add is added as a per-atom force (and since
that is apparently what you want), you cannot dump that as per atom
torque or rotational velocity (omega). this is nonsense. you have a
model of a system of point particles and those have no individual
rotations

in general, the GI-GO (garbage in, garbage out) principle applies
here. you have to improve your understanding of the physics of the
models and the effect and purpose of the LAMMPS commands you are using
to set up a successful simulation. there is no LAMMPS command that can
do this for you (i often wish there was, but this is far beyond the
skill of a C++ programmer to implement it).

axel.

3

the problem is in your lack of understanding of what you are doing and
your lack of care (or understanding) when reading the documentation.
there is little that we can do here about it.

in particular:

  • you are using atom style sphere, but there is nothing in your model
    that requires it (save for fix nvt/sphere which will act as fix nvt,
    since there is no per particle torque generated anywhere)

But in order to have a finite particle with radius 3.0(in my case) I am using atom style sphere.

  • fix addtorque act on the fix group and adds per-atom forces on
    group members, that represents the desired torque. the torque you add
    btw. is invalid for a 2d system
  • you are applying time integration and thermostatting to the same
    particles multiple times. this is very wrong and at least for the
    time integration LAMMPS will warn loudly about it.

I got the point.

  • since the torque you add is added as a per-atom force (and since
    that is apparently what you want), you cannot dump that as per atom
    torque or rotational velocity (omega). this is nonsense. you have a
    model of a system of point particles and those have no individual
    rotations

I am bit confused about the above explanation. Here I want to look into the rotational diffusion of these particles, for that I need to use some finite sized particles. That why I have used a sphere style with a finite radius of 3.0 for this simulation. As per my understanding then it is no longer will be a point particle. If I apply a per atom force it will produce a rotation as well. I am trying to print out this rotational velocity.

4

[...]

But in order to have a finite particle with radius 3.0(in my case) I am using atom style sphere.

you can set the radius to anything you want, for as long as you use
pair style lj/cut, it will be ignored.
from your description of the visualization you want to simulate an
object composed of individual particles. those then don't need to have
a radius themselves. the radius is determined by the geometry of the
particles. the rotation of that is induced by fix addtorque. but that
is all *very* different from a simulation of finite size particles,
which require different a different pair style and where you then can
monitor the rotation of individual particles, which is independent of
your entire object.

- since the torque you add is added as a per-atom force (and since
that is apparently what you want), you cannot dump that as per atom
torque or rotational velocity (omega). this is nonsense. you have a
model of a system of point particles and those have no individual
rotations

     I am bit confused about the above explanation. Here I want to look into the rotational diffusion of these particles, for that I need to use some finite sized particles. That why I have used a sphere style with a finite radius of 3.0 for this simulation. As per my understanding then it is no longer will be a point particle. If I apply a per atom force it will produce a rotation as well. I am trying to print out this rotational velocity.

your understanding is incorrect. if you are not using a pair style
that takes the radius into account, it will still be treated as a
point particle, if the pair style does not compute a per-particle
torque, there will be no per-particle rotation regardless of whether
you use a timestepping fix that will update it. i have pointed this
out to you now multiple times and you have so far refused to react on
it. this meams you need to discuss with your adviser and/or your
colleagues. you need help in *fundamental* understanding of the model
you are doing, and how to interpret documentation of a simulation
software, and that is clearly beyond what you can receive from this
mailing list.

i will ignore further e-mails on this subject from you unless you can
demonstrate that you have significantly improved your understanding.

axel.