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Re: [lammps-users] Thermostating dipoles with angular velocity bias in shear flow

# Re: [lammps-users] Thermostating dipoles with angular velocity bias in shear flow

 From: Steve Plimpton Date: Mon, 6 Nov 2017 10:50:42 -0700

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

On Tue, Oct 3, 2017 at 2:25 PM, Qi Liu wrote:
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 rotj-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

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