[lammps-users] fix_deform with rigid particles

I would like to perform shear simulations of systems containing fluid
particles and solid particles constructed with fix rigid. From the
documentation on fix deform for rigid particles,
"If a fix rigid <fix_rigid.html> is defined for rigid bodies, and remap
is set to x, then the center-of-mass coordinates of rigid bodies will be
remapped to the changing simulation box. This will be done regardless of
whether atoms in the rigid bodies are in the fix deform group or not.
The velocity of the centers of mass are not remapped even if remap is
set to v, since fix nvt/sllod <fix_nvt_sllod.html> does not currently
do anything special for rigid particles. If you wish to perform a NEMD
simulation of rigid particles, you can either thermostat them
independently or include a background fluid and thermostat the fluid via
fix nvt/sllod <fix_nvt_sllod> ."

Can this be explained in more detail? Should one apply fix deform to
all particles, or should just the solvent particle coordinates be
deformed, and what remapping should be used? What does it mean to
thermostat the rigid particles independently? They're rigid particles;
why should they be thermostatted? Since I'm using fluid particles,
should I apply fix nvt/sllod to the fluid and fix nve to the solid
particles and rely on the fluid particles to shear the solid particles?
Does this introduce errors when the rigid particles cross the sheared
boundary without having their velocities remapped?

Thanks in advance,
David R. Heine
Senior Research Scientist
Corning, Inc.
Corning, NY 14831

Dave - I'm going to forward this email to Matt Petersen here
at Sandia and let him respond. He's been running these
kinds of simulations and has addressed the various Qs
you ask. You are correct that you really don't need to
"thermostat" rigid particles.

Matt, can you CC your response to
[email protected] as well?


Here was Matt Peterson's answer:

Hi David. So, to answer your questions; you should apply fix deform to
all particles and remap velocities, use the sllod thermostat for the
solvent only, and *don't* apply fix NVE to the rigid bodies.

The deformation of the box using slodd requires that all particles
(including the rigid bodies) have their velocities remapped when
crossing the sheared boundary. Particles at this boundary "see" the
particles across the boundary moving relative to the shear rate, and
without the remapping would crash into the opposing streaming particles
when they cross. The remapping adjusts their velocity to match the
streaming velocity across the boundary. The rigid fix implies an NVE
integrator, so none is needed for the rigid bodies - slodd should only
be applied to the solvent. The sheared boundaries and velocity
remapping will take care of shearing the rigid bodies, and the sllod
thermostatted solvent should be enough to control any heating due to the
shearing. I've found some instabilities at high shear rates; maybe due
to a poorly chosen timestep, or maybe because the system is started so
far from the assumed velocity profile. Also, take care to adjust the
degrees of freedom if you are using liner rigid bodies. The rigid
documentation explains how an why. And use neighbor modify to exclude
interactions between particles in a rigid body. It wont change the
dynamics, but the pressure and temperature could be wrong. Feel free to
email me if you have more questions.