I am running a test setup in lammps. I am using 4000 atom hcp system with meam potential. System need to be at equilibrium at 10K and for this I am giving initial velocity at 10K and running NVE for short duration of 2ps (My timestep is 1 fs) followed by NPT at 10K for 20ps. I want a configuration where system is at 10K with small pressure.
step 1: NVE for short time
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 aspected and system equalibriated at 5K as we started with initial velocity of 10K and 133 bar of pressure system exhibits
Step2: NPT
fix 1 all npt temp 10 10 0.1 iso 0.0 0.0 1
run 200000
OUTPUT
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 100x timestep and Pdamp of 1000x timestep. I have also changed different Pdamp value also and found that pressure varies from -200 bar to +200 bar.
I can suppress this fluctuation by introducing drag term to Nose-Hoover equation.
for example :
fix 1 all npt temp 10 10 0.1 iso 0.0 0.0 1 drag 1
output
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 100 ps but I can still see fluctuations. How much fluctuation in pressure is accepted as these fluctuation increases with temperature also. Any further insight is highly appreciable