[lammps-users] Diffusion constant

I was trying to reproduce the results of Kremer and
Grest in which they obtain the diffusion constant of a
neat polymer melt with repulsive interactions. Their
system is comprised of 75 polymer chains with 80
monomer beads in each chain. The parameter values are
epsilon = sigma = mass = 1.0, temp = 1.0*epsilon and
density = 0.85 (all LJ units).

They found the diffusion constant to be equal to
4.55E-04 (which in the order of N**-2, where N is the
number of bonds in each chain).

I tried to take the same system and redo it using
LAMMPS to obtain the diffusion constant equal to
2.89E-03. To double check my results, I also used the
fix msd command in LAMMPS, which also gave me the same
answer (2.89E-03).

I am attaching my input file to this email. It would
be great if someone (who has done this before) could
help me out and give any suggestions.


in.prod (1.29 KB)

One thing I notice is you're not using a special_bonds 0 1 1 command
which would be typical for FENE. So you are not getting pairwise
terms for 1-3 and 1-4 interactions. If you still have problems you
can ask Gary directly. His email is gsgrest at sandia dot gov.