force dumping from rigid body problem

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1

Dear Steve,

We define part of system as a rigid body and give a constant velocity to it. Then we dump the sum of the forces on the rigid body during the simulation. Instead of giving an average steady state 0 value, it gives a constant value. That doesn’t make sense, since the whole system is moving together with the velocity given to the rigid body

Please see the below script we used:

units metal

time:ps; distance: Angstrom; energy: eV; force:eV/A=1.6 nN;

atom_style hybrid atomic

boundary p p p

read_data lammps.dat

timestep 0.001

group bot type = 1

group therm1 type = 2

group film type = 3

group tipdown type = 4

group therm2 type = 5

group tiptop type = 6

group tip union tipdown therm2 tiptop

group sub union bot therm1 film

group mobile subtract all bot

group subnve subtract all tiptop

group tem union therm2 therm1

group dump union tip film

pair_style hybrid/overlay tersoff table linear 1000

pair_coeff * * tersoff SiC.tersoff C C C C C C

pair_coeff 3 4*5 table lj.table CC 10.0

fix 1 bot setforce 0 0 0

velocity mobile create 300.0 23456789 units box

fix 2 mobile langevin 300.0 300.0 0.01 699483

compute 1 tiptop reduce sum fx

compute 2 tiptop reduce sum fz

compute new3d mobile temp

fix 13 tiptop ave/time 1 7350 7350 c_1 file tiptop-fx1

fix 14 tiptop ave/time 1 7350 7350 c_2 file tiptop-fz1

dump 1 dump cfg 7350 dump.*.cfg id type xs ys zs x y z id type

thermo_style custom step temp etotal press vol lx ly lz

thermo 7350

thermo_modify temp new3d

Cutting speed: 1.0 A/ps = 100 m/s

#fix 3 tiptop nve/noforce

#fix 4 subnve nve

#unfix 3

#unfix 4

fix 21 tiptop rigid/nve single force * on on on torque * off off off

fix 22 subnve nve

velocity tiptop set 0.2 0 0 units box

run 1000000

Thanks

Zhendong

2

The sum of the forces on the rigid body are
from atoms external to the body. Why should
they be zero? They could be anything. Just
because you are constraining its motion, doesn't
change the forces acting on it. It just ignores them.

Steve

3

Dear Steve,

Let me give more details of this simulation.

The whole system is surrounded by vacuum. Part of the system is defined as a rigid body and made to move at a constant speed. Please see the attached picture. Therefore, the average force on this rigid body should be 0, as there is no friction or dumping forces. However, the result shows a constant force on the rigid body.

Please advise.

Thank you very much for your patience.

Best regards
Zhendong

Picture1.png

4

Dear Steve,

Let me give more details of this simulation.

The whole system is surrounded by vacuum. Part of the system is defined as a rigid body and made to move at a constant speed. Please see the attached picture. Therefore, the average force on this rigid body should be 0, as there is no friction or dumping forces. However, the result shows a constant force on the rigid body.

there *is* friction due to your langevin thermostat.

axel.

5

And why isn't there a force on the red rigid atoms from
the green atoms?

Steve