# Thermostating dipoles with angular velocity bias in shear flow

Dear Lammps users,

I’m currently working on the simulations of dipoles in a shear flow. The shear flow has the velocity in the x direction and vorticity Ω = -0.5 dvx/dy in the z direction. The vorticity makes the particles spin with the flow, so the rotational friction is proportional to the relative angular velocity of particles. Then the 2 friction terms in the Langevin equations are -ζt [vj-v(rj)]​​ in the translational momentum equation and -ζrotj-Ω(rj)]​​ in the angular momentum equation. j is the particle index.

I know I can thermostat the system with a translational velocity bias like in the following:

#for comparison with temperature computations with a bias

compute comp_tTrRot all temp/sphere dof all
compute comp_tTr all temp
compute comp_tRot all temp/sphere dof rotate

fix fix_def all deform 1 xy erate \${xyrate} remap v flip yes units box
compute comp_tTrDef all temp/deform
compute comp_tTrRotDef all temp/sphere bias comp_tTrDef dof all

#for comparison with compute temp/deform
compute comp_tTrProf all temp/profile 1 0 0 y 20

fix fix_ext all efield 0 {Hy} 0 fix fix_nve all nve/sphere update dipole fix fix_lang all langevin {temp} {temp} {damp} \${randomNum3} omega yes
fix_modify fix_lang temp comp_tTrRotDef

My question is, is there a way to add an angular velocity bias to the Langevin thermostat besides the translational velocity bias from “compute temp/deform”? “compute temp/rotate” subtracts center of mass velocity and angular velocity, so it doesn’t satisfy the requirement. Did I miss something important?

Thank you!

Regards,
Qi

No, I don’t think there is currently anyway to do this kind of biasing with rotational DOFs

in LAMMPS. Someone would have to add code for it.

Steve