equilibration of solids


Excuse me if I’m asking at the wrong place. But I’m looking for some help in tensile simulation of a solid beam.
I’m trying to do a bending simulatio of a nano-beam with Aluminium.
I have S S S boundary condition as there nothing periodic about the simulation.
The beam is created in the simulation and I applied the following equilibration and ran it for a 10k steps:


velocity all create 300.0 124365554 dist gaussian
fix 1 Grbeam nvt temp 300.0 300.0 10.0

I observed in any case the length of the beam is decreasing and shape of the beam is getting distorted.
I equilibrated the system because otherwise even if I’m applying a tensile force the beam is moving towards the left until
it reaches an equilibrium length or the force should be too high to overcome the ‘natural’ pull towards the right.
I have two doubts:

  1. Is there any thermo to measure the instantaneous length of any group in lammps like we do for the simulation box (lx, ly, lz)
  2. Can I equilbrate the beam without getting the shape distorted or is it right for the beam to have a decreased legth in equilibration.

I know this is a very much in particular doubt, please ignore it if you find it out of context. I’ve been burning my head on this for quite sometime.
Any help will be really appreciated.

Thank you

A free chunk of material may compress or expand on its
own if that brings it to a lower energy state.

Equal-style variables can use bound(group,xmin) for
example to find the smallest x value of any of its atoms,
So a formula that used xmax - xmin would be the length
of the beam.