[lammps-users] Pressure calculation of water system via stress/atom

Dear LAMMPS users,

I tried to analyse the pressure distribution of a water-containing system.

A test system only contains a water box, and NPT (1.0 atm, and 300 K) runs for 1.0 ns , then another 1.0 ns follows for NVT (300 K). The local pressure is computed from stress/atom command, through adding all stress components inside the bin together and dividing by the -3*volume of the bin. While it shows many negative values in both the NPT and NVT processes, which does not make sense.

As well known, the pressure calculation is not that easy like temperature calculation, while the negative values are really far away from the real cases. Does anyone have experience about the pressure calculation of water-containing system and would you kindly share some suggestions? How can we compute the pressure for water system much closer to the atmospheric pressure?

Pressure computed from virial stress fluctuates and it will fluctuate more if the volume is small.
Thus you need to run a simulation in equilibrium and average over that for a sufficiently long time.