Elastic Example With AIREBO

I’m using the elastic example on a data file for one unit cell of a (12,1) single wall carbon nanotube just to get started. I’m using the AIREBO many body potential.

However the elastic constants I’m getting, specifically, c11 and c33 are very different from one initial reference I’m looking at, which is “J. Lu PRL. vol (79), 1997”.

From what I’ve gathered in the archives, usually the only data to include in a data file when using many body potentials should just be the Atoms section, and that’s what I’m currently using for these runs with the ELASTIC examples.

However in this case, with the ELASTIC example should I include bond and angle data in the data file for the initial configuration?

v/r,

a log is attached.

log.lammps (312 KB)

I'm using the elastic example on a data file for one unit cell of a (12,1)
single wall carbon nanotube just to get started. I'm using the AIREBO many
body potential.

However the elastic constants I'm getting, specifically, c11 and c33 are
very different from one initial reference I'm looking at, which is "J. Lu
PRL. vol (79), 1997".

there are many possible reasons.

From what I've gathered in the archives, usually the only data to include in
a data file when using many body potentials should just be the Atoms
section, and that's what I'm currently using for these runs with the ELASTIC
examples.

However in this case, with the ELASTIC example should I include bond and
angle data in the data file for the initial configuration?

no, why?

axel.

Is the reason for not explicitly including Bond & Angle data when minimizing with the AIREBO, or another many body potential related to the special note on the minimize documentation.

It warns that, “There are several force fields in LAMMPS which have discontinuities or other approximations which may prevent you from performing an energy minimization to high tolerances. For example, you should use a pair style that goes to 0.0 at the cutoff distance when performing minimization (even if you later change it when running dynamics)…” and also that “Some of the manybody potentials use splines and other internal cutoffs that inherently have this problem.”

Is the reason for not explicitly including Bond & Angle data when minimizing
with the AIREBO, or another many body potential related to the special note
on the minimize documentation.

It warns that, "There are several force fields in LAMMPS which have
discontinuities or other approximations which may prevent you from
performing an energy minimization to high tolerances. For example, you
should use a pair style that goes to 0.0 at the cutoff distance when
performing minimization (even if you later change it when running
dynamics)..." and also that "Some of the manybody potentials use splines and
other internal cutoffs that inherently have this problem."

sorry, but this statement has *absolutely nothing*
to do with bonds and angles in your data file.

axel.

Is the reason for not explicitly including Bond & Angle data when minimizing with the AIREBO, or another many body potential >related to the special note on the minimize documentation.

No many-body potential in LAMMPS uses defined bond and angle terms. Whether
you are running dynamics or minimization. The point of a many-body potential
like AIREBO is to allow bond breaking/formation dynamically. Defining fixed
bonds is a different kind of model.

Steve