[lammps-users] simulation box drifting in Z

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1

I have an issue concerning drift in the z direction. I’m simulating a box of water with a deform command to keep the xy boundaries fixed at the desired value (b/c I will be eventually placing it on a surface) and NPT with z pressure NULL NULL. I began equilibration wtih NVE (without the deform) and the water-box behaved as expected however when I change to NPT I’m getting the water box moving in the z direction (drifting), I’ve tried various options but haven’t been able to solve the problem, I’m a bit new to the program so any help would be greatly appreciated!! (Below is my input file)
Thanks in advance,
Jennifer

atom_style full
pair_style lj/cut/coul/long 12
bond_style harmonic
angle_style harmonic
dihedral_style none
improper_style none
kspace_style pppm 1e-6
read_data H2O_slab.lammps
neigh_modify delay 5
fix 1 all deform 1 x final 0.0 46.0 y final 0.0 45.0 units box
fix 2 all npt temp 300.0 300.0 100.0 z NULL NULL 1000 tchain 10 pchain 10
fix 3 all momentum 1000 linear 1 1 1 angular
thermo_style custom step temp pe ke etotal lx ly lz vol
thermo 10
timestep 1
dump 2 all custom 100 dump.lammpstrj id type xs ys zs
restart 10000 H2O_slab
run 20000

2

I have an issue concerning drift in the z direction. I'm simulating a box of
water with a deform command to keep the xy boundaries fixed at the desired
value (b/c I will be eventually placing it on a surface) and NPT with z
pressure NULL NULL. I began equilibration wtih NVE (without the deform) and
the water-box behaved as expected however when I change to NPT I'm getting
the water box moving in the z direction (drifting), I've tried various
options but haven't been able to solve the problem, I'm a bit new to the
program so any help would be greatly appreciated!! (Below is my input file)

jennifer,

please try the deformation without "fix momentum" active.

cheers,
    axel.

3

The syntax for your fix NPT command is wrong. You can't use NULL,
the code is likely reading those as numbers which are 0.0.
If you meant NULL to mean don't control the pressure in z, then
why use fix npt at all? Also, why are you using such large
chain values? I would start simple and not use fix momentum.

Steve