Atom deposition using rebo potential

LAMMPS LAMMPS Beginners
1

Hello everyone,
I am trying to simulate a simple atom deposition model using the rebo potential as shown in the script below. The same script already exist in the examples file when I download the lammps, but with the LJ potential. I have two problems with this:

First problem: The substrate atoms suddenly blow up instead of just vibrating in their position. I tried to lower the temperature of the FIX langevin command and the thermo step but the best result I got is unstable atom vibration until step number 50.

Second problem: The deposited atoms do not interact with the substrate atoms. I tried to increase the deposition velocity to a 100 Angstroms/picosecond just to reach the substrate before the blow up that happens after the 50 step, but the deposited atoms just penetrated the substrate and there is no interaction between the atoms to simulate the deposition phenomenon.

I am fairly new to using lammps so I am hoping that these problems are simple. If you could tell me how I can solve these problems or any other issues , i would really appreciate it.

Thank you very much in advance!

Abdelrahman

units metal
atom_style atomic
boundary p p f

lattice sc 2.0
region box block 0 20 0 20 0 30
create_box 2 box

region substrate block 0 INF INF INF INF 11
create_atoms 1 region substrate

pair_style rebo
pair_coeff * * CH.rebo C H
mass * 1.0

neigh_modify delay 0

group addatoms type 2
region mobile block 0 20 0 20 5 11
group mobile region mobile

compute add addatoms temp
compute_modify add dynamic/dof yes extra/dof 0

fix 1 addatoms nve
fix 2 mobile langevin 1 1 0.001 587283
fix 3 mobile nve

region slab block 0 20 0 20 16 17
fix 4 addatoms deposit 1000 2 1 12345 region slab near 0.5 vz -100 -100
fix 5 addatoms wall/reflect zhi EDGE

thermo_style custom step atoms temp epair etotal
thermo 1
thermo_modify temp add lost/bond ignore lost warn

dump 1 all atom 1 dump.deposit.atom
run 100

2

Do you know any allotrope of carbon that is a bulk system with a simple cubic lattice and a lattice constant of 2.0 angstrom. I know of diamond and graphite but those are not what your input is creating and apparently the REBO potential doesn’t “want” to be in that geometry as well. Hence the blowing up. This is the GI-GO principle in action and not something that can be fixed with a thermostat. As a physically accurate model, it does what “real” carbon atoms would do if forced into such an unrealistic geometry. This has nothing to do with depositing atoms.

Carbon atoms that weigh only as much as hydrogen. That is not going to help with getting accurate dynamics.

3

Dear Axel,

Thank you so much for your comments. I tried to change the crystal structure to diamond and also changed the lattice constant and it worked just fine.