Residual Stress

Dear all,
Hi! I hope you are fine. I have a question?!
Why do we have residual stress when simulation box is relaxed under NVT?


The simulation box can’t be relaxed if you only did NVT runs. NVT forces the simulation box to have constant volume which whatever you’re simulating may not like. If you use NPT with the aniso keyword, you should have negligible residual stresses (and thus a relaxed state).


Dear professor Pisani,
Thanks for your help. But, when I use NPT in this relaxation, my Strain Energy Density in Compression (not in tension) isn’t minimum at first step which is weird.

As my result shows, the minimum value appears in third step. How can I fix it?


Best Regards,

You don’t need to fix it. The difference is only -0.06% from the initial value. That’s small enough to not worry about it.

If this data is from a pure NPT relaxation (no loads), then your average stresses (and the elastic strain energy) will trend toward zero (which is what you want). Negligible stresses are near-zero stresses. Eventually your stresses will look like typical MD pressure fluctuations.

Hope this helps,