[lammps-users] Stress/atom and pressure relationship

Dear Steve / Lammps Users.

I am trying to calculate total stress on my system. As mentioned in the compute stress doc page, I am printing out stress per atom and summing up for all atoms and then for all diagonal terms and dividing by 3*volume. However the value of pressure which is coming is different than the one which is printed out in thermodynamic output. Is it because of the fact that stress calculation doesn’t consider intra-molecular forces while the total pressure calculation does consider all forces.

If that is the case, please put a note just for clarification in that doc page that the value of pressure after conversion is going to differ since intramolecular forces are being neglected.

If that is not the case, what could be the possible reasons. I will highly appreciate if anyone could help me out here…


Yes, no bond contributions. From the doc page for compute stress/atom:

Note that this formula for stress does not include virial
contributions from intra-molecular interactions (e.g. bonds, angles,
torsions, etc). Also note that this quantity is the negative of the
per-atom pressure tensor.