fix deform uniaxil tension questions


I’m attempting to perform uniaxil tension along the x-axis of a polymeric material. Currently my code is:

fix 2 all nve
fix 3 all temp/berendsen {sim_temp} {sim_temp} 100.0
fix 4 all press/berendsen y 1.0 1.0 100.0 z 1.0 1.0 100.0 modulus 20000.0
fix 5 all deform 1 x erate ${srate} units box remap x

I’ve adapted this from one of the tutorials (

I’m using berendsen temperature and pressure thermostats since I use those during the equilibration. I’m wondering if this is the correct way to perform a tensile simulation with LAMMPS? Or, would it be better to use “fix nvt/sllod” with the “fix deform” keyword ‘remap’ set to ‘v’? Additionally, should “fix npt” with ‘aniso’ for ‘y’ and ‘z’ be set, with 0 pressure?

Thank you for your time and insight.

nvt/sllod is for when you are doing a NEMD model with
a liquid that is shear flowing with the deformation velocity.

Use of press/berendsen vs npt aniso is up to you. They do
different things.


Hi Steve,

Thank you for your reply. Would you mind expanding a little about press/berendsen and npt aniso doing different things? I’ve read the doc pages for both, in addition to looking in the cited papers, however, I’m still curious, especially when having the opportunity to question someone with far more simulation experience than myself.


What I meant was that aniso is different than the x,y specification
you were making for press/berendsen. Both npt and berendsen
support either aniso or x,y. Otherwise they are similar, other than
that fix npt has more options, and Nose/Hoover versus Berendsen
are somewhat different algorithms. NH keeps a history of previous
volume/pressures that affect the current change; Berendsen is
more immediate.