brownian motion enquiry

Dear all,

I am studying the files that allow one to perform BD, however, with a single particle, when I plot r^2 vs time, the graph is non-linear. I was wondering whether anyone has seen this problem before and whether I should use fix viscous, even though the langevin fix is supposed to include the damping term (as well as add random collision force)?

Also, I am slighlty confused about the random number seed – if I understand correctly, this is not a gaussian rn generator? I do not have access to the Dunweg paper:

(Dunweg) Dunweg and Paul, Int J of Modern Physics C, 2, 817-27 (1991).

Does anybody have this article?

my input is as follows:

units real
atom_style atomic
boundary p p p
atom_modify map array sort 1 1
restart 1000000 particle.restart
read_data data_particle
reset_timestep 0
group all type 1
dump 1 all custom 100000 particle_ts100_delta1m.dump id x y z ix iy iz
dump_modify 1 format “d .5f .5f .5f %d %d %d”
fix 1 all nve
fix 2 all langevin 1.0 1.0 2.0 424242
thermo 100000
thermo_style custom step temp pxx pyy pzz vol
timestep 1
run 10000000

thanks in advance.

Fix langevin should do BD. You don't need fix viscous.
Fix langevin uses a uniform random # with a different
pre-factor to account for that, which is a faster method
than using a Guassian RN.


Your units are "real"
yet your fix langevin is
"fix 2 all langevin 1.0 1.0 2.0 424242 "
Which is what you usually find in "lj" units where you set your
temperature to 1.0.

Are you trying to do BD at 1K?