[lammps-users] Stress-starin of bulk polymer


I’m working on a polymeric material (described by pcff) and would like to evaluate its modulus by monitoring the stress-strain behavior. The issue is that stress doesn’t show any meaningful trend and only oscillations are observed around the set overall pressure. Below are the key command lines in my input script:

fix 1 all npt temp 300 300 100 y 0 0 1000 z 0 0 1000" #strain along x

fix 2 all deform 1 x erate ${strain_rate_fs}" #where strain rate is 1e-8 1/fs

compute p all pressure thermo_temp
variable stx equal -0.101325*c_p[1] #in MPa

Despite time-averaging of v_stx, it doesn’t seem to be increasing in the course of simulation (2ns with timestep of 0.5 or 1 fs).

It would be highly appreciated if you could advise what I’m doing wrong here, or if there are better ways to approach this (for instance change_box).

Thanks in advance for your inputs.

The first step in cases where you don’t get the expected result is always to visualize the trajectory and see if you notice any unexpected behavior.

You also need to take into account that “soft” materials can have a very different behavior than, e.g. metals, so that significant trends may need larger systems and longer simulations and more averaging.

Have you compared your system size and simulation settings with similar studies of similar materials?


Is your model periodic in X-direction? ‘Fix deform’ won’t work if it isn’t periodic in the corresponding direction.


Many thanks for all your inputs.

The model is periodic in the strain direction (x). And the issue was resolved when I coupled fix deform with fix nvt/nve. As per Eliar’s suggestion, I also made the other two dimensions non-periodic through the deformation, but not sure how extensively it could affect the results.

My system size is comparable to those used in similar studies (500 molecules), however I believe results could be further improved by increasing the chain length. Will try that in my next attempt.

Best regards,