[lammps-users] Thermostating the rigid bodies

Hello Everyone,

   I am trying simulate the aggregation of 64 rigid spherical particles each
consisting of 15 Lennard-Jones atoms. There is no solvent in this system
(explicit or implicit). The snapshot the initial configuration is attached with
this e-mail. The LAMMPS documentation states following about fix rigid:

  "This fix updates the positions and velocities of the rigid atoms with a
constant-energy time integration, so you should not update the same atoms via
other fixes (e.g. nve, nvt, npt, temp/rescale, langevin)."

  "Note that this fix uses constant-energy integration, so you need to impose
additional constraints to control the temperature of an ensemble of rigid
bodies. You can use fix langevin for this purpose to treat the system as
effectively immersed in an implicit solvent, i.e. a Brownian dynamics model. Or
you can thermostat the atoms of an explicit solvent directly."

   As in my case I don't have any solvent and all the entities are rigid, how
can I thermostat the system? Any help in this regard will be greaty appreciated.


Gaurav Pranami

Graduate Student
Chemical Engineering Dept.
Iowa State University

InitConfig.bmp (706 KB)

You have an implicit solvent (no background particles), so you
can try the fix langevin to see if it equilibrates your system.

In LJ units, something like this has worked for me:
fix 2 all langevin 0.5 0.5 1.0 904297


Hi Steve,

      I found this old post which has to do with my question. Does it mean
that I can use "fix rigid" and "fix langevin" simultaneously to equilibrate
the system of rigid bodies with a heat bath?
     The documentation I read says one should not update the "fix rigid"
with other fixes including "fix langevin". So I'm not certain of it.


I amended the doc files to clarify this. You can use fix langevin
with fix rigid to do a Brownian dynamics style simulation, where
fix langevin plays the role of an implicit solvent that thermostats
the collection of rigid bodies. The attached script does this
effectively for a collection of bucky-balls, each of which is a rigid
body of 60 particles, a la the work on patchy particles of Sharon
Glotzer's U Mich group - see Nanoletters 2003,2004.


in.bucky (471 Bytes)