Active Stress contribution in LAMMPS

I am using LAMMPS version 29th September 2021. Applying fix langevin, fix nve/asphere and fix propel/self to study a very simple active particle model, I would like to study the stress in the system, including the contribution due to activity.

From what I understand in the documentation, when you use fix propel/self, a virial contribution to pressure can be added, but no contribution to stress is added, is it? Should I use “compute stress/atom” command to get this contribution? Or is it not implemented in LAMMPS yet?


In LAMMPS pressure is -stress/volume. And stress itself is the virial stress plus a kinetic stress contribution. So your understanding is incorrect.

Compute stress/atom does not compute anything (in contrast to the name), but rather triggers that the virial contributions from various force computations are collected on a per-atom basis (this is normally not done for efficiency reasons) and then make it available for output or further processing.

Fix propel/self has a rather detailed discussion of how stress/pressure can be computed (and what not), plus some references to related publications. For additional information about pressure/stress, you need to look at the documentation of compute pressure and compute stress/atom and fixes in general as well as relevant text books.