[lammps-users] Doubt on "fix deform" command

Dear lammps users,
I am using “fix deform” command with “fix nvt” to induce a tension simulation. The simulation box is cuboid in shape and has polymer and nanotube. In “fix deform” documentation it is mentioned that for volume to be conserved, we need to provide volume style (also it is realistic). But I could observe that while calculating the poison ratio, always I got a value of 0.5 . So if we use deform command, irrespective of the material, poisson ratio remains constant. Am I right regarding this? Also, is “fix deform” the appropriate command to apply a tension/compression for a material (with PBC)?

Dear lammps users,
I am using “fix deform” command with “fix nvt” to induce a tension simulation. The simulation box is cuboid in shape and has polymer and nanotube. In “fix deform” documentation it is mentioned that for volume to be conserved, we need to provide volume style (also it is realistic). But I could observe that while calculating the poison ratio, always I got a value of 0.5 . So if we use deform command, irrespective of the material, poisson ratio remains constant.

it has to be constant because by applying the volume constraint in fix deform you are enforcing dimension changes in all directions and do not have a response from the material (it has no choice but must follow what you enforce).

Am I right regarding this? Also, is “fix deform” the appropriate command to apply a tension/compression for a material (with PBC)?

yes, fix deform is the suitable option for straining bulk systems.

axel.

Thanks for your reply. Let me try to understand here. If I use fix deform for material with an actual poisson ratio of 0.3, I will get a Poisson ratio of 0.5 because I am enforcing volume conservation. Now if I need to calculate Poisson ratio of material by inducing tension, “fix deform” is not the correct way. Am I right? Can you suggest a method in LAMMPS to predict poisson behavior?

Thanks for your reply. Let me try to understand here. If I use fix deform for material with an actual poisson ratio of 0.3, I will get a Poisson ratio of 0.5 because I am enforcing volume conservation. Now if I need to calculate Poisson ratio of material by inducing tension, “fix deform” is not the correct way. Am I right?

no. you are not paying attention. fix deform is the correct way, but enforcing volume conservation is not.

Can you suggest a method in LAMMPS to predict poisson behavior?

if you want your system to relax/respond to the straining according to its intrinsic properties, you must not use fix nvt, but fix npt, and couple the dimensions that you are not deforming to a barostat.

axel.