[lammps-users] TIP4P NPT non-hybrid: volume increases

Dear all,

I am running an NPT simulation of box containing 256 H2O molecules. As others have reported before in this list, I find that the volume keeps increasing throughout the simulation time.

It was suggested not to use the pair_style hybrid command, which I am not using. Any clues where this problem may come from?


PS: I’m copying here a part of the code that I’m using:

units real
dimension 3
boundary p p p
atom_style full

read_data waterbox256.lammps05
#read_restart waterbox256_tip4p.min-npt.rst1

pair_style lj/cut/coul/long/tip4p 1 2 1 1 0.125 7.5
pair_coeff 1 1*2 0.000 0.000
pair_coeff 2 2 0.16275 3.16435
bond_style harmonic
bond_coeff 1 450 0.9572
angle_style harmonic
angle_coeff 1 55 104.52
kspace_style pppm/tip4p 1.0e-5

neighbor 2.0 bin
neigh_modify delay 0 every 1 check yes

timestep 0.5
run_style verlet

I just did a short TIP4P simulation of water and get no difference in
the pressure
when running lj/cut/coul/tip4p by itself or under the pair_style hybrid. So an
NPT model would give no difference.

I know Ahmed (CCd) has done extensive modeling of water with NPT and TIP4P and
seen good agreement with published results for pressure, volume, etc.
So I'll let
him comment.

The only outstanding bug I know of in this area is when using SHAKE with
rRESPA - the pressures are off. TIP4P uses SHAKE, but you are not using

So as far as I know, anything you found on the mail list is old news. I assume
you are using the current version of the code?