# viscosity calulation

Dear Lammps users,

I am trying to calculate viscosity of an amorphous system at 300 K (after quenching) using the equilibrium method with the Green-Kubo formula but I get I completly wrong result about 1 (Pasec) while a value higher than 10^12 (Pasec) is expected.

I used the script that from the viscosity examples and I converted the units from metal to SI. I also read previous messages with similar problems but I didn’t figure out how to solve my problem. Do you have any suggestion or idea of what maybe the problem?

Thanks a lot in advance ,
Alex

Here is the input that i used

# Sample LAMMPS input script for viscosity AAAAAAAAA

variable T equal 300
variable V equal vol
variable dt equal 0.001
variable p equal 200 # correlation length
variable s equal 1 # sample interval
variable d equal \$p*\$s # dump interval

# convert from LAMMPS real units to SI

variable kB equal 1.3806504e-23 # [J/K/** Boltzmann
variable bar2Pa equal 100000.0
variable A2m equal 1.0e-10
variable ps2s equal 1.0e-12
variable convert equal {bar2Pa}*{bar2Pa}{ps2s}*{A2m}{A2m}*{A2m}

units metal # Angstrom, eV, ps, bar
boundary p p p # Periodic boundaries in all dimensions.
atom_style atomic # Pair potentials.
atom_modify map array

For the value to be that badly wrong suggests a units problem, but all of the usual caveats that you’ve probably found in the mailing list still apply: does your correlation function converge well (you can visualise it to check), etc.