Viscosity Question


Hi, I am trying to calculate Viscosity using SLLOD NEMD method on LAMMPS. I found a formula that gave visc =

/strain rate. Which pressure tensor (Pxy, Pxz, or Pyz) am I supposed to use for

if we set up the strain using:

fix 1 all nvt/sllod temp {simtemp} {simtemp} 100.0

fix 2 all deform 1 xy erate ${strain} remap v

And does anyone have a resource that explains how to know?

Thanks in advance,


Undergrad Research

Hi Tyler,

This type of measurement is explained in many introductory text books on molecular dynamics simulation. Good references include:

· Computer Simulation of Liquids by M.P. Allen and D.J. Tildesley

· Understanding Molecular Simulation: From Algorithms to Applications by Daan Frenkel and Berend Smit

· Statistical Mechanics: Theory and Molecular Simulation by Mark E. Tuckerman

See if you can get any of these books from your campus library. You definitely want to understand the underlying physics of what you are doing before you attempt to study it in simulation. The nature of your question makes me think that you don’t fully understand how the fix deform command works, or what the components of the pressure tensor mean. I’d recommend you also study the concepts of stress, strain, and linear response theory.

Hope that points you in the right direction.


Thomas C. O’Connor

Ph. D. Candidate

Johns Hopkins University

In examples/VISCOSITY these is also an in.nemd.2d

script and README which explains its usage for a simple