Calculating Viscosity

Dear LAMMPS users,

I am using the latest version of LAMMPS. I’d like to calculate viscosity using NEMD method. The problem is when I am trying to calculate the viscosity of a polymeric liquid under different shear rates, I see shear thinning while I know that this polymer should has a Newtonian behavior(polymer = PEG400).

(shear thinning: viscosity decreases by increasing shear rate)
(Newtonian behavior: viscosity is constant under different shear rates)

And about my velocity profile; despite I use the real units, Is it ok to haven’t desired smooth velocity profile because of very low number of shear rates?

I have sent the input script and data file. I would appreciate if any one can give me some advice about my mistakes.

thanks in advance,

Eliar (580 KB) (2.19 KB)

I am not sure what is telling you that a “PEG melt” is always exhibiting Newtonian behavior, do you have a source for this? A more important question to ask though is what force field are you using? Have you validated that force field against relevant properties? Has anyone else used this force field and what have they observed? An improper force field will make you observe all types of weird things.

The PEG simulations community is quite particular about their force fields because PEG is quite a unique polymer. Many people use the CHARMM-c35r force field. Choosing to model PEG with something other than this will likely raise questions down the road.

Just some food for thought.