[lammps-users] freezing orientaion??

Dear lammps users,

Is it possible to freeze the orientational degrees of freedom of the molecules in
my simulation cell? I don’t want to change the orientation of the molecules but
ofcourse the translational degrees of freedom will be there. if it is possible could
any of you give some suggestions. Any comments or suggestions are highly
appreciated. Thanking you in advance.

kind regards

If you have flexible moleclues, then LAMMPS does not
treat them as molecules, but as atoms which feel forces.
Thus there is no simple way to compute and zero the
torque on an entire molecule. You'd have to write a new
fix to do that.


Hi Bhaskar,

If only a few (not many) molecules need to be frozen, you can treat each of them as a rigid body (by using “fix rigid” command) and modify the source code (fix_rigid.cpp) a little bit. In src/fix_rigid.cpp, line 650-652, line 852-854, you can find the following:

torque[ibody][0] = all[ibody][3];
torque[ibody][1] = all[ibody][4];
torque[ibody][2] = all[ibody][5];

I think you need to assign 0.0 to these torque components:
torque[ibody][0] = 0.0; //all[ibody][3];
torque[ibody][1] = 0.0; //all[ibody][4];
torque[ibody][2] = 0.0; //all[ibody][5];

Then compile the modified LAMMPS and run.

Good luck!