Wrong viscosity is printed for a simple water box

Hi everybody,

I have a simple water box and hereby followed the LAMMPS documentation to find the pressure tensor components and viscosity. Attached files are input, data and output files.
The viscosity for water molecules is about 1 mPa.s ; however, I don’t know what’s the problem here that I get zero viscosity (plz see “logequi2”??

  1. Another problem that I get is about calculating pressure tensor (plz see ptensor1). As you can see pressure tensor components have the order of 1e9 which leads to the 1e10 mPa.s for the viscosity! So, these two approaches doesn’t lead to the same result.
    Thanks for your comments,
    logequi2.txt (8.5 KB)
    ptensor1.profile (20.0 KB)
    solvate.data (1.9 MB)
    solvate.in (2.7 KB)

Please see the examples/VISCOSITY folder for some examples of different methods to determine viscosity.

As indicated by the uppercase name of the folder, this is an advanced topic, so perhaps you should work on something else first to become more experienced and competent in your simulation skills.
What you didn’t mention, but what is crucial is to investigate the convergence of your results. I am fairly certain that your simulations are simply far too short for reliable results (and that includes the rather reduced equilibration).

If you need help beyond creating a syntatically correct input file, you need to look for a proper tutor that has experience in computing viscosity from MD simulation. This is ultimately more an issue of how you are doing your research in general rather then how to use LAMMPS.