Equlibration problem: Structure collapsing during NVT calculation

I am a beginner in LAMMPS and I wish to do MD simulation for graphyne Sheet to see its stability at different temperaure.I have made this input after consulting the documents and executed it on a data file consisting of a graphyne sheet made out of carbon atoms only and plotted the graph for total energy, potential energy, kinetic energy, temperature etc with respect to number of step.

During the NVE calculations, structure retains. Then I made a NVT simulation for increasing the temperautre stepwise till 300 K and observe the changes in parameters. But During the NVT The structure is compromised and all the atoms collapse. So I am doing something wrong which I can’t figure out. Please help me.

I have used the Airebo potential as it has been used by similar works on graphyne.( https://doi.org/10.1007/s00339-020-03623-1 )please tell me where I am doing mistake? and also suggest a better Input. I will be very thankful to you

The Input file is as follows:

units real
dimension 3
boundary p p p
atom_style atomic
read_data Lammps_graphyne.data
pair_style airebo 3.0
pair_coeff * * CH.airebo C

thermo_style custom step etotal pe ke
thermo 1000
neighbor 2.0 bin
timestep 1
fix 1 all nve
dump graph all xyz 50 out.NVE.000*.xyz
run 20000
unfix 1

thermo_style custom step etotal pe ke temp
thermo 1000
neighbor 2.0 bin
timestep 1
velocity all create 300 5432373 dist gaussian
fix 2 all nvt temp 300.0 300.0 100.0
#dump graph all xyz 50 out.NVT.xyz
run 20000
unfix 2

I am not able to understand why the carbon based structure collapses as the temperature is increased to around 130 Kelvin.

Harkishan Dua
Department of Physics
Assam University, Silchar

the AIREBO potential file bundled with LAMMPS requires using “metal” units.

it always helps a lot to read the documentation first.