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I am running a test setup in LAMMPS. I am using $4000$ atom hcp system with MEAM potential. The system need to be at equilibrium at $\pu{10K}$ and for this I am giving initial velocity at $\pu{10K}$ and running NVE for short duration of $\pu{2ps}$ ($\pu{1fs}$ timestep) followed by NPT at $\pu{10K}$ for $\pu{20ps}$. I want a configuration where system is at $\pu{10K}$ with low pressure.

Step 1: NVE for short time

Input

velocity all create 10 4928459
fix 1 all nve
run 20000

Output

    Step      Temp         TotEng       Press
   18500    4.9325929   -6191.5587    135.54744
   19000    5.0476045   -6191.5587    134.10653
   19500    5.0523007   -6191.5588    133.15733
   20000    5.1187478   -6191.5588    132.81736
   20010    5.0694554   -6191.5587    133.90782

Final output is as expected and system equlibrated at $\pu{5K}$ as we started with initial velocity of $\pu{10K}$ and $\pu{133bar}$ of pressure.

Step 2: NPT

Input

fix 1 all npt temp 10 10 0.1 iso 0.0 0.0 1
run 200000

Output

    Step      Temp         TotEng       Press
  217000     10.04384   -6186.4403    157.98486
  217500    9.7292229   -6186.5088   -11.700501
  218000    9.7728786   -6186.3937   -92.777014
  218500    10.165017   -6186.3155   -69.765655
  219000     9.875577   -6186.5188   -10.970014
  219500    9.9095766   -6186.5424    74.030269
  220000    10.251927   -6186.3377    182.70663
  220010    10.259622   -6186.3333     76.85412

Here I used Tdamp of $100\times \text{timestep}$ and Pdamp of $1000\times \text{timestep}$. I have changed different Pdamp value also and found that pressure varies from $\pu{-200 bar}$ to $\pu{+200 bar}$. (Note for Non-LAMMPS users, the damp parameters give the amount of time for that property to relax).

I can suppress this fluctuation by introducing drag term to Nose-Hoover equation:

Input

fix 1 all npt temp 10 10 0.1 iso 0.0 0.0 1 drag 1

Output

    Step      Temp         TotEng       Press
   39000    10.074096   -6186.3848   0.88364453
   39500    10.001291   -6186.3747   0.29228989
   40000    9.9810361   -6186.3857  -0.12452421
   40010    9.7588549    -6186.392  -0.91715159

But I want to suppress these fluctuation in pressure only using real ensemble (with no drag). I tried to increase further run to $\pu{100 ps}$, but I can still see fluctuations. How much fluctuation in pressure is acceptable, as these fluctuations increase with temperature also? Any further insight is highly appreciated.

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    $\begingroup$ This might answer your question: mattermodeling.stackexchange.com/questions/4625/… $\endgroup$
    – Vasista
    Jul 1 at 8:28
  • $\begingroup$ pressure fluctuations are normal, runs of only 100 ps are not. $\endgroup$
    – Wesley
    Jul 2 at 11:52
  • $\begingroup$ @AdupaVasista I understand the problem that instantaneous pressure is not well defined but I am searching practical way to reduce this because for metallic system which have lower peierls stress (stress needed to move dislocation) can be affected by these fluctuations and dislocation can move without external force $\endgroup$ Jul 2 at 12:27
  • 1
    $\begingroup$ @AdupaVasista This came up in the queue of tasks for volunteer close voters to evaluate. Does Pranav's answer to your question (i.e. understanding that pressure fluctuations are normal, but looking for a practical way to reduce them) satisfy you to retract your close vote? Pranav: your question's body asks "how much fluctuation is acceptable?" But you seem to also be looking for a way to "suppress" fluctuations. Can you pick one question for this post, and perhaps ask the other one separately if you find you still need help? $\endgroup$ Jul 2 at 13:08
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    $\begingroup$ @NikeDattani I retract the vote. $\endgroup$
    – Vasista
    Jul 3 at 10:39