low density of carbon nanotue

Dear lammps users,
Hello! I’m trying to equilibrate single wall carbon nanotube system. However, I got a lower density system compare to the reference. I used the following script

units real
dimension 3
atom_style full

neighbor 2.0 bin
neigh_modify every 2 delay 0 check yes page 100000

pair_style lj/charmm/coul/long 10.00 10.10
kspace_style pppm 1.0e-4
bond_style harmonic
angle_style charmm
dihedral_style charmm
improper_style harmonic
kspace_modify gewald 0.195
atom_modify sort 0 0.0

read_data lammps_data01

fix 1 all npt temp 273.15 273.15 100.0 iso 1.0 1.0 1000.0
#fix npt temp 300.0 300.0 100.0
#special_bonds lj/coul 0.0 0.0 0.5 angle yes dihedral yes

timestep 1.0

thermo_style custom step temp pe etotal press vol density
dump 2 all xyz 1000 xyz.file-run10
dump 3 all custom 1000 dump_vmd-run10.lammpstrj id mol type x y z ix iy iz
thermo 1000

restart 1000 data.lj-1.as.benze-run10 data.lj-2.as.benze-run10

run 2000000

It’s a 5 ncell-(6,6) single wall carbon nanotube, but I got a density around 1.2g/cm3. I built the structure with no-charge system and applied kspace_modify command. I also visualized the trajectory. There is some space between carbon nanotubes, and the CNT looks only vibrating after the density reached to 1.2. Can anybody give me some comments to me? Any help would be appreciated.



Is the pressure of the system equilibrating at your requested pressure?

If you start with a less dense or more dense system, is it re-equilibrating
to the same density you are seeing now? If so, then the NPT
is working properly. Your model is what is producing the density
value you are seeing.


Two comments:

  • kspace convergence of 1.0e-4 is OK for fixed size cells, but we’ll converged pressure requires a tighter convergence like 1.0e-6.
  • you a using isotropic cell change. This is unlikely to be suitable for nanotubes. Their reaction to pressure should be different in different directions, that is anisotropic.