High fluctuation at lower strain rate.


I am trying to find out stress strain plot for a amorphous polymer cell at 300K. Am using “fix deform” to applying strain rate of 1e-4 along x direction. The size of my simulation cell is 250 x 250 x 250 angstroms.

The strain rate am using is very high compared to the experimental strain rate. Here the bonded energy contribution is high compared to nonbonded energy, but it should be the other way.

So, I tried to simulate with a strain rate of 1e-7, the stress strain plot is so fluctuating I couldnt get the modulus values, or even the pattern of the plot is not found.​

I have crossed checked the parameters and am using charmm forcefield which was used for polymers from literature.

Can anyone get me some idea why there is large fluctuation in the stress strain plot.

Following is the command I am using


​fix 1 all npt temp 300 300 5 y 0 0 50 z 0 0 50 drag 1
fix 2 all deform 1 x erate 1e-7 units box remap x​


The “erate” has a unit of (1/time units), not 1/second. So 1e-4 may not be as small as you thought.


Stresses generally go down with strain rate. So your stress curve could be comparable with the background noise if the strain-rate is slow enough or if your system is small.


Hi, Santhosh. You didn’t give us much information to go on, so it’s hard to be of much help. I would attack this problem by first trying to duplicate a stress-strain curve from published work (if any) using the SAME simulation protocol employed in that work. (same system size, equilibration procedure, etc.)