Dear all,

To compute viscosity, I am running NEMD by deforming the box and using SLLOD

integration.

One thing I noticed is that for LJ fluid in triple point condition, **without**

**shear**, the average pressure tensor (Pxy Pxz Pxz) is always around 0.0001, while

fluctuation is around 0.1. When I introduced shear, the fluctuation level is still

around 0.1. I just wonder if this high fluctation is normal. If yes, how to estimate

the deviation when computing viscosity?

Thanks in advance!

best wishes

Zhenlong

Pressures fluctuate, often by very large amplitudes. The

off-diagonal terms are worse b/c they are typically

small, even in shear for a fluid. You can use fix

viscosity (Muller-Plathe) method which doesn't

require monitoring the pressure.

Steve

Thanks, Steve.

I know velocity exchange is another way to calculate viscosity. But as for viscosity calculation by shear flow, I still want to make it clear how to do it within LAMMPS.

- how to estimate the error bar for viscosity, considering the high magnitude fluctuation

of stress tensor?

- To say, if the shear flow is introduced in x direction, and the velocity profile is

therefore generated in y direction, Pxy is required to calculate the viscosity. With available

options in LAMMPS, we can get Pxx, Pyy, Pzz, Pxy, Pxz, Pyz. I just wonder if the Pxy is exactly the one we need, or it is the average of Pxy and Pyx (they are different in this case).

Thanks

with best wishes

Zhenlong

Re: error estimate - that's a general Q, not really a LAMMPS Q, so I

won't attempt to answer.

Re: Pxy - LAMMPS does not compute Pyx. I think you want Pxy for

this viscosity calc. And I think you should always be able to setup

your simulation geometry so that Pxy (not Pyx) is what you want.

Steve