Dear All,

I am going to do uni-axial tension, uni-axial compression, simple shear , and loading/ unloading (cyclic loading) for glassy polymers using Lammps with period boundary conditions along 3 dimensions.

I have 2 questions:

(1) During the deformations mentioned above, I would like to output stress-strain information not only on the surfaces of the simulation box, but some inter-planes within the box, so I have to divide the box into parts. Is it allowed in Lammps? If so, what is the command for the it?

(2) During applying the deformations, for example, uni-axial tension, I need to measure the temperature rise due to the deformation, so I can’t use NPT ensemble since I don’t want to keep the temperature constant. But I do have to control the stress (pressure) on the

lateral directions, they should be zero in uni-tension tests. What ensemble do you think is suitable for my tests? Can pressure control be used independently from the ensembles? Or maybe I shouldn’t use any ensemble at all, just need to apply the deformations and control some components of the stress.

Thank you for your time and advise.

Lili Zhang

PhD student, Mechanical and Materials Engineering

University of Nebraska-Lincoln, Lincoln, NE 68588-0526

There is a compute stress/atom command which can
calculate stress on a per-atom basis. It can then
be spatially averaged with the fix ave/spatial command
in various ways. There is no mechanism in LAMMPS to
calcualte "stress across some aribtrary interface", like
a plane you define. But that formulation of stress is
equivalent to other definitions. If that's really what
you want, you'd have to post-process it from snapshots.

You can use the NPH ensemble if you don't want to
control temperature. But you probably just want
NVE and to use the fix deform command to apply
the strains. It has many options; see the doc page.

Steve