[lammps-users] equilibration of water to specified pressure

Dear all

I have changed the potential from Charm to cut/cool/long . Even then there is large fluctuation in pressure.I had gone through all the parameters i have used again and again. But couldn’t locate any error.Now i am attaching my input file. Please help me to get a solution for this.
Thanks in advance

in.waterNPT5 (2.09 KB)


If you are seeing the instantaneous pressure change in the range of hundreds of atmospheres, that’s actually fairly typical of MD simulations. So long as the long-term average is correct, and the fluctuations are decreasing with time (constant magnitude, divided by increasingly larger number of samples), I don’t think there’s actually a problem that can be fixed.


Thank you for your suggestion.In my case the fluctuations are not decreasing with respect to time. I have calculated the long time average (average of 1000 time step) of pressure . That is also found to be fluctuating. Also i have tried by changing the time step from 1fs to 2fs. No change in fluctuation happens even after 10 million time step.

#Input file for SPC/E water

boundary p p p
restart 50000 restart

#Simulation Units andSimulation Type

units metal
atom_style full

#Reads configuration from file

read_data waternpt.txt
#read_restart restart.*

#Mass relative mass of atoms

mass 2 15.99943
mass 1 1.00794
group oxygen type 2
group hydrogen type 1
set group oxygen charge -0.8476
set group hydrogen charge 0.4238

#Define potential type

pair_style lj/cut/coul/long 10.0 12.0
kspace_style pppm 5.0e-5
kspace_modify mesh 36 36 36 order 5

#Lennard-Jones Parameters

pair_coeff 2 2 0.0067371579 3.166
pair_coeff 1 1 0.0000000000 0.000
pair_coeff 1 2 0.0000000000 0.000
bond_style harmonic
bond_coeff 1 19.512 1.0
angle_style harmonic
angle_coeff 1 2.3848 109.47
group water type 1 2
velocity all create 373 234324 dist gaussian mom yes rot yes
neighbor 2.0 bin
neigh_modify delay 0 page 200000 one 4000
#minimize 0 0 1000 10000
thermo_style custom step pe ke etotal temp press vol xlo xhi ylo yhi zlo zhi

#Applies or “fixes” NVT MD

fix 1 water shake 0.0001 20 500 b 1 a 1
fix a all nve

#Dumps output files and thermodynamic properties

dump 1 all atom 1000 dump.waterNPT5
thermo 500

#Timestep is 1 fs (0.001 ps)

timestep 0.002

#Number of time steps (reduced)
compute mobile all temp
#fix b all nvt temp 373.0 373.0 1.0

fix b all langevin 373 373 1.0 48279
fix_modify b temp mobile
run 100000
#unfix b
#fix a all temp/berendsen 373 373 100
#fix_modify a temp mobile
#run 500000
#fix b all langevin 373 373 100.0 48279
#fix_modify b temp mobile
compute baro all pressure mobile
fix jsb water press/berendsen iso 1.013 1.013 1000
#fix jsb water press/berendsen xyz 1.013 1.013 1000
fix_modify jsb press baro
fix c all ave/time 1 1000 1000 c_baro file press.profile

#unfix jsb
thermo_style custom step pe ke etotal temp press vol xlo xhi ylo yhi zlo zhi
run 10000000