To compute viscosity, I am running NEMD by deforming the box and using SLLOD
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!
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.
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).
with best wishes
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.