Subject:

That's not the correct formula for any potential.

It's not position (r) invariant, meaning if you

translate the box you'll get a different answer.

See the compute stress/atom doc and compute

pressure doc pages for how LAMMPS computes

the pressure.

Steve

Let me restate what I just posted more accurately.

The formula is correct, but you have to be careful

about how you calculate it. In a periodic system,

you can't just use F dot r. It isn't translationally

invariant, e.g. if you shift the atoms wrt to the

box, you get a different answer.

On the citations page of the WWW site, see

the paper by A Thompson if you want details.

The compute stress/atom doc page tells

you how you can compare 2 different ways

of calculating the pressure within LAMMPS.

They should agree for AIREBO and every

other potential.

Steve

Thank you very much for your explanation.

C.M.

Dear Dr. Plimpton,

In the compute stress/atom doc page it is said that "...F1 and F2 are the

forces on the 2 atoms resulting from the pairwise interaction". Then, to

calculate the stress using that formula, I need to know the force on each

atom due to each pairwise interaction they encounter. Then the pressure

cannot be calculated from only knowing the total force acting on each

atom. Is that correct?

Best regards.

C.M.

The issue is periodic boundary conditions. The forces you

need to calculate pressure may actually be on ghost atoms.

LAMMPS takes care of this, but by the time you dump forces

per atom to a dump file, it's too late. You don't know if the

forces were on the real atom or its ghost images.

Steve