friction coefficients (xmu) in granular simulation


I am Ashish Orpe, a research scientist working at the National Chemical Laboratory, India. I am using lammps to carry out simulations of granular particles while draining down a silo. I use gran/hooke/history as the pair_style.

I have following question

Is it possible to have a scenario with part of particles with a certain friction coefficient (xmu) and the remaining with other friction coefficient (xmu)?

As per my information, pair_style hybrid option used as following is not possible

pair_style hybrid gran/hooke/history 2000000 NULL 158.113883 NULL 0.001 0 gran/hooke/history 2000000 NULL 158.113883 NULL 0.5 0

Note, everything is same in the above commands except the xmu value (5th argument), so effectively, it is not really a hybrid pair_style

Further, if using the pair_style hybrid command, the arguments are needed to be given with pair_coeff for a specific set of particles which is also not allowed within granular style

– Ashish

From the pair_gran.html doc page:

For granular styles tFor granular styles there are no additional coefficients to set for
each pair of atom types via the “pair_coeff”_pair_coeff.html command.
All settings are global and are made via the pair_style command.
However you must still use the “pair_coeff”_pair_coeff.html for all
pairs of granular atom types. For example the command

pair_coeff * * :pre

should be used if all atoms in the simulation interact via a granular
potential (i.e. one of the pair styles above is used). If a granular
potential is used as a sub-style of “pair_style
hybrid”_pair_hybrid.html, then specific atom types can be used in the
pair_coeff command to determine which atoms interact via a granular

So I think you can do what you are asking w/ 2 different pair style commands

in pair hybrid. You can also look at the LIGGGHTS simulator,, which is built on top of LAMMPS.

I know it has options for setting these interaction coeffs differently

for different kinds of particles.