Thank Dear Axel for your quick respond.

The older and newer versions are lammps-64bit-20160720 and

lammps-64bit-20160902 respectively.

I solved the problem by calculating angular velocities by adding the

following lines to the input file (only the z-component is shown here):

compute COM_Cu Cu com

variable dx atom x-c_COM_Cu[1]

variable dy atom y-c_COM_Cu[2]

variable dz atom z-c_COM_Cu[3]

variable Lz atom (-vx*v_dy+vy*v_dx) # Angluar

momentum in z direction

compute Lz Cu reduce sum v_Lz

variable Iz atom (v_dx^2+v_dy^2) # Moment of

inertia about z-axes

compute Iz Cu reduce sum v_Iz

variable W23 equal c_Lz/c_Iz # angular

velocity of Cu around z-axes

variable wz_NP equal omega(Cu,z) # angular

velocity of Cu around z-axes (by omega function)

compute w_NP Cu omega/chunk CHUNK # angular velocity of Cu

around z-axes (by omega/chunk compute)

The results show complete agreement between the results of the added lines

and both "compute omega/chunk" and "function omega()" which both show

exactly “zero” in the newer version. As you can see the angular velocities

are small but are not "zero":

step

c_w_NP[2][3]

v_wz_NP

v_W23

0

1.46E-06

1.46E-06

1.46E-06

100

2.44E-06

2.44E-06

2.44E-06

200

7.17E-07

7.17E-07

7.15E-07

My original input file is very big so I send a small one to demonstrate the

problem.

Unfortunately, when my simulation box gets higher than a certain value, all

older versions of LAMMPS crash and show this message: "Imp-mpi.exe has

stopped working"

Fortunately, the newer version has not this problem.

Best regards