Simulation box of water in NPT

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1

Dear users,

I’m trying to simulate a box of rigid SPC water molecules in NPT ensemble.
However during the process, the pressure always changes dramatically
from hundreds to thousands. Besides, the total energy is negative.

Below is relevant part of the input script. Any help or guidance would be appreciated.

units real
dimension 3
boundary p p p
atom_style full
bond_style harmonic
read_data spcwater.data
velocity all create 298 12432 dist uniform
fix 1 all shake 1e-6 500 0 m 15.994
fix 2 all npt temp 298 298 500 iso 3173 3173 500
thermo 100
timestep 0.05
run 200000

Best regards,
Fan

2

Dear users,

I'm trying to simulate a box of rigid SPC water molecules in NPT ensemble.
However during the process, the pressure always changes dramatically
from hundreds to thousands. Besides, the total energy is negative.

so what? you have a rather incompressible medium
and your system size is likely not very large, so the
tiniest changes in the box dimensions *will* cause
large pressure fluctuations. read through the
documentation. read a book on MD!

axel.