Compute viscosity using Muller - Plathe method

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1

Hi all,

I want to simulate the viscosity evolutions of water with the increase of surfactant concentrations using Lammps. Since I need to keep simulating box orthogonal, I adopt Muller - Plathe method and “fix viscosity” in Lammps. In the manual, this fix should not use a thermostat, so I chose “NVE” after a 5 ns NPT equilibration, but temperature are still fluctuating dur this process, and finally increase 10 degC.

(1) Is it necessary to control temperature during the swaping process?

(2) I want to know the viscosity changes of water, so I only need to swap the momentum of water, rather than the whole system, right?

Regards,
Jun

2

You are using a flexible water model, correct? Not constrained, e.g. via SHAKE.
The fix viscosity doc page explains why this is needed.

It’s possible your system is not fully equilibrated before switching to NVE. Or
that if you run too long, or are too aggressive in your swaps that NVE will
see a temperature rise. B/c you are artificially assigning big velocity
changes to atoms. You might also try limiting your swaps (via the fix viscosity group)
to only the heavy O atoms.

Steve