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

From: Qi Liu <iamllqq@...24...>
Date: Tue, 3 Oct 2017 16:25:10 -0400

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!