[lammps-users] pressure fluctuations in NPT

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



Why is your timestep so tiny? ~1.0 fs seems more

Pressure fluctuations are normally huge in liquid
phase molecular simulations. And more so for smaller

Also, your time constants on your npt command seem
small. Instead of:
fix 1 all npt 293 293 10 xyz 1.0 1.0 10 drag 2.0
Perhaps you could try something like:
fix 1 all npt 293 293 100 xyz 1.0 1.0 1000
Or even:
fix 1 all npt 293 293 1000 xyz 1.0 1.0 10000
(maybe a bit on the large side for the time constants,
but may be necessary with your tiny system)

I hope this helps!