I am working with NPT ensemble on a polymer chain (well not really, 104 atoms: carbon and hydrogen). My timestep is 0.01 fs. When I run it for 200,000 timesteps, pressure seems to settle around 1atm which is the defined pressure. However when I extend the run to 500,000 timesteps, pressure makes big fluctuations between 124 and -160 atm. which is either too big or small, which sound unrealistic.
I am not sure what is happening after 200,000 timesteps. The amplitude of the oscilations around 1 atm gets bigger towards 500,000 timestep on pressure vs timestep plot.
My fix command in the input file is:
fix 1 all npt 293 293 10 xyz 1.0 1.0 10 drag 2.0
Thanks for any feedback