Green-Kubo Viscosity Parameters - Near Critical State

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1

Hello LAMMPS users/developers!

I’m trying to calculate viscosity and thermal conducticity for a system of 100,000 atoms using Green-Kubo approach with LAMMPS.

System is near critical state.

I’ve experienced great divergences between calculated results and experimental data.

I imagine I’m having problems with integration parameters for time-correlation functions.

My calculation steps are:

  • create atoms with random scheme (Argon atoms with an LJ potential)
  • minimize to eliminate overlaps of atoms. Parameters: 1.0e-14 1.0e-16 1000 10000
  • perform a NVT run to equilibrate temperature and pressure as desired: 100,000 steps
  • calculate time correlations: 100,000 steps (same code as manual)

Considering time correlations i’ve used the folowing parameters:

Nevery = 1 (fixed)
Nrepeat = (50, 100, 200, 400, 800) - I used some different values to get some feeling for this parameter
Nfreq = 1000 (fixed)

I’ve obtained the following results:

Nrepeat Viscosity (µPa.s)
50 9.000
100 18.184
200 36.553
400 73.289
800 146.761

Experimental result from NIST database: 29.661 µPa.s

It seems that so as I double Nrepeat, Viscosity also doubled. From Nrepeat definition in LAMMPS doc page, I expected a convergent behaviour for viscosity. Definitively it doesn’t happen.

Do you have any idea that could help me in getting better results?

Best regards

Jakler Nichele
Military Institute of Engineering
Defense Engineering Graduate Program
Rio de Janeiro - Brazil
jaklernichele@…3041…

2

From the numbers you list it seems clear

that you need to normalize them by Nrepeat to
get a consistent value. Not clear whether
fix ave/correlate should be doing that or you
just need to do it yourself. Are the doc pages
for the command inconsistent with what it
is printing to its output file as you vary Nrepeat?

Reese - maybe you have a comment?

Steve

3

Dear Steve,

It seems to me that there are some inconsistency related to Nrepeat. Or I didn’t understand manual correctly…

For thermal conductivity the example presented in doc page does not results in the indicated value.

Sincerely yours,

Jakler Nichele
Military Institute of Engineering

Defense Engineering Graduate Program
Rio de Janeiro - Brazil

4

Does anybody have any idea?

Thanks a lot.

Jakler Nichele
Military Institute of Engineering

Defense Engineering Graduate Program
Rio de Janeiro - Brazil

5

Haven't had a chance to look at the normalization
issue yet - stay tuned.

Steve