Help me in Viscosity Calculation using Growmacs

Hello everyone,

I hope you’re all doing well. I’m currently working on a GROMACS MD project where I’m trying to calculate viscosity and understand how an organic molecule is extracted from water using a deep eutectic solvent (DES). I’ve read a few recent papers, and it seems like the Green–Kubo method and shear viscosity are good approaches, but I’m still a bit confused about how to proceed properly.

So far, here’s what I’ve done:

  • Used the CHARMM36 force field

  • Ran 200 ns simulation (MD run)

  • 20 ns NVT

  • 100 ns NPT

  • Then added the organic molecule and ran another 100 ns

  • The system looks stable and even shows a nice layering in the trajectory.

My general workflow has been:

Energy minimization → NVT → NPT → Production MD

I have a few questions and would really appreciate your suggestions:

1. What should I do next for viscosity calculation?

2. Which system should I focus on?

Is it better to calculate viscosity:

  • Separately for each system (pure water, pure DES, organic in water, mixture),

    or

  • Only for the final mixture where extraction actually happens?

3. .mdp settings confusion, do I need to change the .mdp files and run the file again ?

4. What should my workflow be now?

If anyone has worked on similar systems (DES, viscosity, or extraction studies), I’d really appreciate your guidance. Even small suggestions would help a lot.

Thanks in advance!