relative velocity distribution for velocities in event of collision

Dear Lammps users,

I am using lammps (unpacked 29Nov 2011) to simulate granular materials. My goal is to determine the relative velocity distribution between two particles in the event of a collision (first overlap). I use the granular package and the pair style gran/hooke/history. The relative velocity at the first collision event can be identified if (touch[jj] == 0), so the particles have not been in contact before. How or where can I save the relative velocity for this pair in a vector (histogram) and update the histogram when the next collision event takes place?

Thank you very much for your help and suggestions!
Best regards
Max Eggersdorfer

LAMMPS doesn't log such a statistic. I think you'd have
to instrument the pair style yourself and create the histogram.

It's possible you could write a fix to do this which would
examine the neighbor list and touch variable, but that
might be too "late" (after the collisioin) and too expensive to do every step.


Just had another thought on this.
The pair_gran_*.cpp files define
a single() function that returns extra
quantities about the pairwise interactions,
e.g. the tangential forces. This is
invoked via compute pair/local, so that
you can for example, dump these values
on a per-pair basis. You could possibly add
extra fields in the single() method
to store the relative velocity.

The you could post-process the dump file
(see the dump local style) to do
the histogramming.

Or maybe you could even use fix ave/histo to
do the histogramming on the fly.