# Why does a particle rotate

Dear all,

I am simulating a nano-particle heating (but still as a solid state) in a vacuum using the following conditions:

units metal
atom_style atomic
lattice fcc 4.123
region box block -60 60 -60 60 -60 60 units box
region particle sphere 0.0 0.0 0.0 40 units box
create_box 1 box
create_atoms 1 region particle
pair_style eam
pair_coeff * * /A.fcn
velocity all create 300 1178591
neighbor 1.0 bin
neigh_modify every 1 delay 10 check yes
timestep 0.001
thermo_style custom step pe press temp vol lx ly lz pxx pyy pzz
thermo 1000
dump myDump all custom 1000 /.../filename*.dump id type x y z
fix 1 all nvt temp 300.0 900.0 0.1
run 5000000
unfix 1
fix 1 all nvt temp 900.0 900.0 0.1
run 1000000
unfix 1

Thus, it is a sphere (R=40 A) in a big box, and I think nvt or npt does not change the results too much. However, I find out the particle was rotating during temperature increasing or when temperature is constant. Now, I am trying to calculate its rotation velocity, but I do not know if there is any criterion about rotation speed. Furthermore, I still do not understand why it would rotate. Thank you for any suggestions in advance.

Best regards,
Leo

Dear all,

I am simulating a nano-particle heating (but still as a solid state) in a vacuum using the following conditions:

units metal
atom_style atomic
lattice fcc 4.123
region box block -60 60 -60 60 -60 60 units box
region particle sphere 0.0 0.0 0.0 40 units box
create_box 1 box
create_atoms 1 region particle
pair_style eam
pair_coeff * * /A.fcn
velocity all create 300 1178591
neighbor 1.0 bin
neigh_modify every 1 delay 10 check yes
timestep 0.001
thermo_style custom step pe press temp vol lx ly lz pxx pyy pzz
thermo 1000
dump myDump all custom 1000 /.../filename*.dump id type x y z
fix 1 all nvt temp 300.0 900.0 0.1
run 5000000
unfix 1
fix 1 all nvt temp 900.0 900.0 0.1
run 1000000
unfix 1

Thus, it is a sphere (R=40 A) in a big box, and I think nvt or npt does not change the results too much. However, I find out the particle was rotating during temperature increasing or when temperature is constant. Now, I am trying to calculate its rotation velocity, but I do not know if there is any criterion about rotation speed. Furthermore, I still do not understand why it would rotate. Thank you for any suggestions in advance.

it rotates because you assign velocities from a random distribution.
those are not likely to be sum up to a net zero translational or
rotational velocity. the thermalization does not pay attention to
this, because it only works on a per atom basis and compute
temperature for the whole (space frame) system.
you have to use the "zero" option of velocity to remove translational
and angular momentum from the initial velocity distribution.

you still have to be very careful with your simulation parameters,
because if your errors become too large, you can pick up residual
translation or rotation again and you'll be simulating a "flying ice
cube", i.e. the actual temperature of your nano particle in the
particle reference frame will be lowered by the energy that went into
the translation and rotation.

axel.