Density and Box length fluctuations

I am using LAMMPS reaxff force field for water. During NPT equilibration system density and box length are decreasing and increasing respectively from 1 g/cm^3 to 0.0012 g/cm^3 and 18.01 angstrom to 64 angstrom.


fix 2 all npt temp 300.0 300.0 100.0 iso 1.0 1.0 100.0

timestep 0.25
dump 2 all custom 1 traj_npt.lammpstrj id type x y z vx vy vx ix iy iz
thermo_style custom step temp pe press vol lx ly lz density
thermo 100
thermo_modify flush yes

run 5000000

write_data npt.dat

Any suggestions or comments??



It is difficult to help with the small amount of information you provided. I am quasi certain that the error is originating in another part of your code. Also reaxff is a tricky force field to use, is there a reason you are using it? If not I would start with simpler force field like spc/e or tip4p.


1 Like


  1. I am using reaxff because I want to see the proton transfer reactions later.
  2. I am already familiar with the simpler force fields like SPC/E and tip4p.

Could you please recommend some tutorial or guide me more about reaxff force field?


Unfortunately no, but I will write a tutorial for reaxff in the future as it seems to be a force field of interest. In the meantime, in the LAMMPS folder there are examples that are working, I would start from there if I were you :