# Infused Rigid Particles moves diagonally

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.

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

Regards,
Salehin

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.

​axel.​