Will fix deform allow free edges to shear?

LAMMPS LAMMPS General Discussion
1

Hello LAMMPS community,

Thank you for taking the time to read my message and for your interest in this topic. I’m currently working on a model where I apply deformation in the x direction while using an NPT integrator to allow natural contraction along the other two edges of the box.

I have a question regarding shear: if shear occurs when I apply tension to the box, would it be accommodated by the boundary conditions I’ve set? I’m using periodic boundary conditions (p p p) and have activated “change_box all triclinic” as I would for simulating pure shear.

Please let me know if you need any more details. I’m happy to provide additional information.

Best,
V

2

Hi @victor.cc,

without much information on the material you are trying to deform, it is hard to predict what can happen when doing shear. But if you turn the triclinic mode on, boxes angles are allowed to vary to relax shear stress. The resulting variation will also depend on the option you pass to your fix npt integrator as you can impose shear stress using the tri or xy ... xz ... yz ... keywords.

Note that periodic boundary conditions are mandatory if you want to control the related pressure in a given direction/angle, but not for triclinic shapes of simulation boxes.

1 Like
3

Hi @Germain

Thank you so much for your response. I changed my npt integrator from this:
fix constant_pressure all npt temp 1.0 1.0 0.01 x 0.01 0.01 0.1 z 0.01 0.01 0.1
to this
fix constant_pressure all npt temp 1.0 1.0 0.01 x 0.01 0.01 0.1 z 0.01 0.01 0.1 xy 0.01 0.01 0.1

and it worked! I will post any follow-up questions on this. Thanks a lot :slight_smile:

best,
V